Ending a mortgage

Hi,
I am looking to potentially move out of my flat (shared ownership 30%) and rent somewhere else. I am just looking at what penalties there would be for ending the mortgage early, within a fixed term, to see if it is possible.
One query is to do with an overpayment I made on it last year.
It's possibly a silly question, but how would I see that overpayment? Will it be as if I just paid more on the deposit initially?
Any help would be great!

Thanks,