Open the AI file in AI (or another application that can reliably open AI files) and change them. There's no other means.
I don't know of any other application that can reliably open and save Illustrator files, except Illustrator of course.
Open the AI file in Acrobat Pro. If the file has been saved "the normal, totally usual and proper way" (*), it contains a PDF preview image which you can edit with Acrobat Pro. You may get a warning that all changes made that way will be lost as soon as the file is opened again in Illustrator.
(*) I somehow felt the need to emphasize that. Just the other day I received a large batch of Illustrator files of which someone must've thought: "oooo I wonder what happens when I switch off a couple of these options ... nothing? oh well, away with it.", and next thing I knew I had to open all of the files (with The Real Thing™ -- Illustrator) and re-save them again.
Depends on what is in the AI file.
CD does a normally good job unless there are meshes and a particular type of transparency (I forget which one). I usually don't get "fancy" AI files. Stock & Trade, bare-bones types of files. these open in a few of the applications I use daily. That said, I never do it unless I need to return a format other than AI--because, well, why? I always try to use the originating application with the exception of older EPS files. As a lot of the daily stuff I do is screen printing, and because most every screen printer I deal with use CD, at some point the art has to go into CD.