Keep in mind that, since the Agreement is client specific, it is located under the CONTACTS TAB. So, you need to go to CONTACTS TAB > Client Name > Agreements tab and click on the "Edit" option in the drop-down menu to the right of the agreement. Make your modifications throughout the agreement wizard tabs and save.
Important note: While you can edit a client agreement, it is essential to keep in mind that editing the billing settings of an agreement with automated invoicing after the client signed the agreement with the initial billing settings will stop the automated invoicing. So, try to make all the editing in the agreement before the client signs it. If you really need to change the billing settings in an agreement after the client signed, you can switch it to manual invoicing. Or, if automated invoicing is important for you, then you can cancel the agreement and create a new agreement with the changed billing settings.