so i have a speedrun in Fire mario 1-1 SMB1 and i think i got a run that i got a better time on. Should i just submit it or do i have to get my previous record deleted before i can submit the better time?

Just submit it. Previous PBs are nice for historical purposes.

The leaderboards automatically update with your best time.

(Unless the board is set to show obsoleted times by default, but I don't really see why anyone would have that set as such.)

I've seen it for sparsely-populated boards where differences in platform or route make for drastically different runs (i.e.sure PC is always faster, but who has the best Playstation run/using a knight is always faster, but who has the best mage run)