How do I Import/Replace a save file for undertale?

[Sunspirals]

So, I was doing a genocide route (This is my first route by the way) and when I got to the mettaton NEO boss fight, i hadn't cleared the area. After I had killed Mettaton, i went back to Hotlands to finish grinding the monsters only to find that i couldn't encounter them anymore. So I did reaserch and found out what I had done wrong. I just need to know how to replace my save file with a different one so I can finish grinding the monsters, fighting mettaton and moving on. P.s, I've seen instructions on how to replace saves but all the one's i have seen are unclear.