You're better off just downloading one
While it's theoretically possible to edit a save file (which is apparently actually split over 4 different files), it would require someone to do a lot of research on every possible variable inside the files, and might be further complicated by the presence of validation checks inside the save data. This may or may not already be done by someone, but even if it is, it would likely still be an arduous task setting all the flags exactly how you want them.
Instead, you'll probably have better luck finding one uploaded online for the particular part of the game you want, especially such a significant point as requested in the question. This Reddit post offers many save files from multiple points throughout the game, on both the Pacifist and Genocide routes, along with instructions on how to customize your name and Spare color.
From the comments on your question, it also sounds like you're currently at the point in the game just after
Chara destroys the world at the end of a Genocide run.
Simply wait for a few minutes on the black screen that appears when you load the game until you're given an option to restore your ability to play the game again. Alternately, avoiding the consequences of a Genocide run entirely, which would also get you out of that state, is described in the question Can I ever play Undertale again normally?