How do I reenter a Pin from last year that I need to fix?

What kind of PIN are you needing to fix?    The self-select (SS) PIN you used last year to e-file is not recorded anywhere, but you don't need it if you have last year's AGI number.   You create a new self-select PIN each year that you use to e-file, but you can use the same self-select PIN each year.   You will enter you SS PIN or last year's AGI when you go through the FILE section and actually transmit your return.  

Return to the FILE menu to enter your SS PIN (or last year's AGI).   You will also enter a self-select PIN for this year's return. 

What is the difference between this year's self-select PIN that I am creating now and the self-select PIN that I used last year? 

The self-select PIN and Identity Protection (IP) PIN are 2 different numbers; not everyone has the IP PIN.