There is no formal way to accept or reject changes. However you can restore to a previous version of the document:
1. Scroll up to that version in the document conversation.
2. Select Revert (this will undo all changes that were made after that time).
All changes made by collaborators are documented in the thread, so if you just want to one or two things that someone did, you can view the changes that were made and choose to change them back.
Learn more about the changes that show up in a thread.