You can use tickets to achieve this functionality.
Configure ticket on the assignment shape which is in the previous stage and call setTicket activity in the current flow action's post-processing activity and pass the ticket as a parameter to go to the last process of the previous stage.
GoToPreviousTask activity- will only help if we have previous assignments and also if we are in the same stage/Flow.
Tickets- Will only help you if you are in the same flow or for the child/Parent flows.
I can suggest one work around solution for this if you only want to go to last process. on the click of button try adding a flag and in the previous flow add a fork shape initially and jump to the last assignment when ever that flag condition is true.
And FYI, there is no OOTB way to achieve this functionality if you are using process flow.