First, you must find your Undertale save folder.
This is usually
C:/users/YourUsername/Local Settings/Application Data/UNDERTALE,
but it might be different for you, who knows.
It should contain at least a file called
and a file called
file0, unless you haven't saved your game yet, or the
save was deleted.
Once you have found where your existing save data is, you can load it into Flowey's time machine to edit it.
You don't have to load an existing save though, you can select a preset instead at the top, or use the default preset which is set by default.
Just edit the various fields to your liking. Some fields have a [?] next to them, you can hover over that with the mouse to get some information about the field.
Once you have edited the data to your liking, you can save it using the save button. Just overwrite your existing file0 and undertale.ini.
If you erased your world, you should see a file called
If you sold your soul, you should also see a file called
Just delete these files using your file manager.
If you are using the Steam version of the game, these files might be backed up and restored by the game on startup. In that case, you should delete the Steam data for UNDERTALE in addition to the system_ files, but I have no information on how to do this.