editing a texure in openGL

I work on a drawing software.
I load TGA files which are only characters outlines, and I want to paint this texture.
So how can I edit video memory, for changing pixels colour value for this texture ?
Is it possble ? Is there another way to achieve ?

Keep a copy of the texture in CPU memory, modify it, and update the texture on the videocard with glTexSubImage2D (faster than glTexImage2D, when the texture already exists)

you may also use a devIL library www.devil.org