to edit your save file?

If i already have a near perfect pokemon and i just want to change 1 or 2 things about it?

Also, does the GTS check if a pokemon is legit or not? I got traded one which was obviously hacked but it's nothing unrealistic like 9999 stats, it's just a carbon copy of an event.

i know pokebank will check but if it's gone through gts can i also confirm it's legit and safe to use?