Where can I find who changed a value in a record?

Any action taken or changes made to a record is recorded in the Activity section.

You can see the history of changes made to a record in the Summary View > Activity section of a record.

Follow these steps to see the updates: 

1. Log in to the CRM.
2. Go to a module's List View.
3. Click on the record, for which you need to check for updates.
4. Go to the Summary View.
5. Go to the Activity section.
6. Click on Filters.
7. Enable the Updates checkbox. 

The Activity section displays all actions taken on this particular record.