i´ve just done a sub 3 (true pacifist) and its invalid cuz in an idiot and didnt read the timing start and end point, and i want to do another run later.

It means starting from a clean slate, and deleting any existing save file. Assuming you're on Windows, the location depends on if you're using the Windows or Linux build of the game.

If Windows: C:\Users(You)\AppData\Local\UNDERTALE If Linux: C:\Users(You)\AppData\Local\UNDERTALE_linux_steamver

If there are files in either, delete them all, then you can properly do your run.

ok tnx