You might be able to customize Workspace to rename the Complete button but I'm not sure about that.
However the easiest way might be to ensure that your User Service has a User Action called Submit. If the User Service has a User Action then that action gets displayed in lieu of Complete.
Hope that helps,
How do I see the the User Action in the User Service? I am still using LiveCycle ES.
Hmmm, I seem to have gotten ahead of myself. I appologize.
For the StartPoint you cannot rename the Complete button (unless you customize and, as I said above, I'm not sure about that).
For tasks, you can initialize the task with route names (Routes and Priority property panel of the user service) and then ensure that you have a route called 'Submit'. If the task goes somewhere then that route can be called 'Submit'. If the task is not routed to another user then you can create a route called 'Submit' that merely goes to a SetValue tasks that does nothing.
That's probably a roundabout way of doing what you want to do but it's the only option I can think of.