How to fix a broken LEGO Star Wars Corellian Defender set
How to fix it depends on how broken it is - if you've got a few large sections that need to go back in it might be easier than if they have been broken down to the individual elements.

The first step is to ensure you still have the instructions - if not, they should be available on LEGO's site: Building Instructions and search for "75025" (there are two books so make sure you get both 1/2 and 2/2).

With the instructions to hand, you should then be able to work out what you need to remove to re-attach the parts that have come off - I find that taking a few additional elements off is often enough to add back the parts and I can then rebuild the additional elements.

In the worse case scenario you need to establish which step in the instructions best represents the bulk of the remaining model and then disassemble everything else back to that point.

Alternatively, invent a new ship with the pieces ;)