General:
* Alter
* Change
* Adjust
* Adapt
* Amend
* Correct
* Improve
* Transform
More specific:
* Revise: To make changes to something, especially to improve it.
* Edit: To make changes to a text or piece of writing.
* Customize: To make something specific to individual needs or preferences.
* Reconfigure: To rearrange or change the structure of something.
* Reform: To change something in order to improve it.
* Transform: To change something completely, often into something better.
Informal:
* Tweak
* Fix
* Tinker with
* Revamp
* Overhaul
The best synonym will depend on the specific situation. For example, if you're talking about making small changes to a document, "edit" would be a good choice. If you're talking about completely changing the way something works, "transform" might be a better option.