Every Opportunity and Person record has a Change Log section.
Use this section for details of changes on the Opportunity or Person record:
Date and time of the change
User who made the change (This can be a client if changed via the Client Centre)
Record Identifier (This denotes related records such as Tasks)
Field Name
Old Value
New Value
If you need to undo or revert some changes use the Old Value column and manually alter the field value on the record itself.