Hi, carolina30,
People switch their PhD project all the time within the first few months when they realise that the project is a poor fit or if they feel that there are problems with supervision.
Do you have scholarship? You may wish to see if the scholarship can transfer to the new project. If yes, normally, supervisors love it if you bring your own fund and this can be seen as a plus when you want to switch. Tell the new supervisor that your current project is a poor fit and you realise that your interests did not align with the project. You could also say that the project description and reality is very different (is this true?) when you started work on it. Just be frank and they normally will understand. You will also need to convince your new supervisor that you will finish the project under him.
Good luck!