The Unapprove option is available on Approved transactions to users that have the associated 'Unapprove' Permission.
These permissions can be added to custom user roles or individual users. The Permission Access report can be used to determine which user roles or users already have these permissions assigned. For more information, see User Setup and Security.
If a user has the associated 'Unapprove' Permission an is viewing an 'Approved' Transaction but does not see the 'Unapprove' button, the Transaction is in a closed period. Learn more about closed periods here.
Reopen a Closed Period
To unapprove the transaction, reopen the period so it can be unapproved and edited. To reopen the period, follow the same instructions as closing a period in the link above.
For the unapprove button to be available, the Closed Date field on the Legal Entity record for the necessary Legal Entity needs to be later than the date of that transaction.
If your Organization utilizes Advanced Close, this field will be hidden. Rather, navigate to the Fiscal Year Maintenance Screen and open the closed period. Refresh the desired transaction and the 'Unapprove' button should appear.