After doing mechanical engineering.what to take aircraft maintenance or aerospace engineering?

Exactly as it says aircraft maintenance you'd maintain aircraft, power plant systems, air frames, avionics systems etc. Aerospace deals more with the designing of aircraft, spacecraft and their components.

I would say: choose wisely. I agree with the other commenters that you should select the subject that you find most interesting. The problem is that when it comes to college/university programs, it's really hard to know what you're genuinely interested in until you spend at least one year in that program. So you should ask yourself, "what are all the aspects of Civil Engineering that I find boring?" List as many as you can on a sheet of paper, then ask someone who's taking Aerospace Engineering if anything you listed can be found in Aerospace Engineering. Based on the number of matches, you can make a better assessment of whether or not AE is actually a better option for you. Of course, there may be many matches but you may still decide to transfer to AE, and that's fine too, this is just a cost-benefit analytical tool that may ease the decision making process. From what I understand, transferring from Civil to Aerospace Engineering is also more challenging particularly because the math and physics used in AE are much more complex than those found in CE. This is something you'll have to prepare for. I don't know about your college, but at mine, students have to spend the first two years studying various math and science subjects, and only specialize in AE starting in their third year. In other words, you'll have to prepare to study many other subjects before delving into any actual AE stuff. You really have to think hard, and find out all that you can about the program to which you want to transfer, regardless of where you want to transfer. It's one thing to think that you like something, and another to actually like it. It would be really unfortunate, having spent time and money transferring, and spending time in another program, only to discover that it's not right for you after all.