How difficult is it to improve upon bad grades. Currently, I am getting Cs and Bs, I would like As. Am I trapped in school mode by thinking it should only take one semester to get an A?

The best way to improve your grades is to pay really close attention to the feedback received and to modify your work accordingly.

Read more and think more.

It depends how far behind you are and also how well you have studied subjects which are pre-requesites for your current modules.

Focussing on grades is, ironically enough, part of a cultural problem we have in the UK.
If you are excited by your subject, keen to learn, prepared to obsessively hunt down answers to everything you don't understand, seek out endless problems to solve and put in the hours, then your grades in exams will be so high you won't need to worry about getting an A because it will happen as a by product of what you are doing, Studying should not be a chore, it should be relatively easy as long as you love what you are doing. You of course need a little bit of ability and sometimes a bit of luck in exams too but the above really is the bare minimum you need. i have not met anyone who was struggling who didn't show deficiency in one of those things I have listed.

As a kid I used to be in your position. I was scoring C grades at best and regularly failing exams. The approach I detailed above changed everything overnight. My subsequent grades topped 80% with many over 90% and I stopped worrying about getting As.