SPOILERS:

I finished Legion's loyalty mission and realised that uniting the Heretic Geth with the Main Geth kills a ridiculous more Quarians in Mass Effect 3, I want to have peace and I can do that, yet my last save was literately after Legions loyalty mission, if there's someway to reverse this, i'll be grateful.

If not, well more Quarians will die than their need to be so RIP.

TL;DR Can I reverse/undo the mission I just previously did.

You cannot undo missions unless you have an older save before your decision. You still have the possibility to save quarians in me3, but without going into too much detail you will just have to be very careful with your choices involving tali and quarians.

I've found a way to make Legion Unloyal (save editing) but is there a way I can undo his loyalty mission through save editing?

@seeker3331: You can do stuff like that through all the variable tables in the Save Editor. Google stuff to find the number associated with each quest in the tables.

Yeah, you can save Tali and the Quarians and also the Geth.

This is why I make saves every 30 seconds. I hope you figure it out!

@clairvoyantvibrations: Thank you , You saved me a lot of time and disappointment.

@seeker3331: No problem!