The unapprove option is available on approved transactions to users that have the Unapprove Security Role. For more information on security roles click here.
If your user has the Unapprove user role and you are viewing an approved transaction, but do not see the Unapprove button, then that means that the transaction is in a closed period. For more information on closed periods click here.
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 you would 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 page (circled below in orange) for the legal entity of the transaction needs to be earlier than the date of that transaction.