The 'Unapprove' option is available on 'Approved' Transactions to Users that have the associated 'Unapprove' Permission.
If your User has the associated 'Unapprove' Permission and you are viewing an 'Approved' Transaction but do not see the 'Unapprove' button, the Transaction is in a closed period. Learn more about closed periods here.
Reopen a Closed Period
If you would like to unapprove the transaction then you would need to reopen the period in order to be able to unapprove and make changes to it. 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.