A paper of mine was accepted to a low-tier conference recently, and not indexed by sources such as IEEE, Scopus, etc. I submitted the paper because I wasn't really confident with the paper; but now I am, and I believe that the paper may benefit from conferences that are indexed by good sources such as IEEE, Scopus, ISI, etc

Should I withdraw the paper and send it to somewhere else?

Any advice is welcome!.

Ask your supervisor for advice. You need someone who understands the etiquette of your particular discipline to advise you whether this is viewed as unprofessional behaviour that would damage you. In my own field any student behaving like that would acquire a terrible reputation very fast, and we don't even count conference proceedings on the rare occasion they exist as real publications.

I agree with bewildered. Messing conference organisers around because you don't think their conference is 'good enough' for your work any more isn't very polite and could cause you longer term damage. If you've extended the work since the original submission, you could always try to submit the extended work to another conference, though only if you have the intention of attending. Don't forget though that conference proceedings are 'worth' less than full journal articles. If you really want your work out there and to have an impact, focus on writing full papers.

If I'm not crossing a line, it seems to me that you've opened a lot of threads of late querying a number of aspects of the supervisor-student relationship and expectations, and what to do about publications etc. Although you will of course find help here, I'd strongly suggest putting some effort into trying to develop a support network in your own department or field who can direct you to answers that might be more specific to your particular PhD journey. It sounds like your relationship with your supervisor is a little bit tense on matters such as publication - it really is worth trying to make this relationship work as well as possible. You don't have to like him/her, but you do need to find a way of utilising this relationship to your best advantage and least stress.

Actually, I already did ask the supervisor regarding the conference a long time ago; they never replied to my queries regarding the matter. .

I usually ask people around in the department of course; but I would also ask somewhere else just to get an expended view. So the "effort" is really not the issue; it's just that I'm not getting the answer I'm seeking.

There's nothing wrong with retracting a paper from a conferences (as long as you haven't registered it i.e. payed the fees); you only to need to inform the chair in order to do so.
I posted the post to ask whether or not I should retract and send somewhere else, and see if someone else has the experienced doing it.

But your original idea could be leaked...

I retracted one paper recently.
However, i have a similar paper accepted by another conference.

Ouch, yes. That's a good point.

Fair enough. It might still be worth seeing whether there are informal/formal networks of support at your uni outside of the department where you can get local guidance. For example, our faculty run workshops on pretty much every aspect of writing and publishing papers, and the training people have plenty of experience with all sorts of aspects of the process and the queries that arise.. People like that at your uni are worth getting to know.

If your paper has been already been accepted then bear in mind that:
(i) selection of your paper may mean that someone else missed out on getting to go to the conference. This is why it's also a little unfair when people submit to conferences if they have no intention of going.
(ii) the organisers will be assuming that you or one of your coauthors will be there to present it. So retraction could mess up the schedule.
(iii) conferences aren't just about proceedings. Though it's nice to nice to get something in print, the primary benefit of getting to go to a conference is the face to face interaction with other members of your research community - to get feedback on your own work and to see what other people are working on. Just because the proceedings aren't indexed doesn't mean you won't get excellent feedback for your work at a particular conference. Equally, even if a conference *is* indexed, you might for some reason not have the most beneficial time there.

If it hasn't been accepted yet, then yes, you could probably retract it without causing some of the above issues, though you might still annoy the reviewers if they've already read it.

I'd disagree with this. It could be a highly unprofessional thing to do, and could earn you a bad name in the wider academic community. Think very long and carefully before you do this. It isn't just about whether you 'can' do something, but whether you 'should'.

And I would echo the advice that if you really want to get your work out there you should be looking at academic journals, not conferences, as a means of publication.

It is possible to have good reasons to retract a paper.

One conference has overwhelming number of papers submitted, but another similar conference has comparatively lesser...
In fact, it helps to reduce the complexity in planning for one conference, and it supports the other conference which has fewer participants.