I would like to know how do those 2 words, stimmt and so, when used together, mean keep the change?
In German, "so" has different meanings:
- like the English "so" - use it to exaggerate "du bist so schlecht"="you are so bad"
- it can also be used to talk about a current state of something "behalte es so"="keep it so"
"Stimmt" means "right".
So when you say "It is right so" in English, you mean "keep the change of it/keep it like that" as you finally got the answer right, so you can keep that final change (for example).
Same in German - when you say "right so", the literal translation is "stimmt so" so when you say "keep the final change of the document/keep the document like that" which means "the document is right so", you can say "das Dokument stimmt so".