Take a look at my Pokémon Red - Gen. II Graphics Patch!
Play through the original Pokémon Red we all know and love with huge improvements such as all new sprites from GSC!
No more pixelated "grainy" back sprites!
It's kind of like a sneak-peak to alot of the features included in my main hack, Pokémon Prototype!
Leave your feedback in the hack's thread!
And while you're at it, tell your friends!
Thanks a bunch!
It's pretty much pointless to come listing tools that allow you to do such things so I'm not going to do that (cause you won't learn anything that way).
Depending on what you want to do (edit trainers, wild pokemon etc.), you've to find the data in rom another way but if you're for example editing wild pokemon (I assume you use GSC Wild Pokemon Editor for this), pick up the offset it shows when pokemon in certain area are edited and go to that offset with a hex editor.
There change the level value (=byte) of the wanted pokemon to one of your choosing. Can be anything between 0x00 and 0xFF (or in decimal, 0 to 255).