Modifying Textures during runtime (possibly using FBOs)

Hello,

I would like to find a way to modify Textures during runtime. Things like flipping, or rotating. While trying to find a solution, I ran into FBOs! I was wondering, could I make an FBO, render the Texture in a new way, then export the FBO’s Texture as a result?

What is stopping me here, is I have no idea how to make an FBO that is not the size of the screen, what with viewports and all.

How would I go about doing this, with or without FBOs?

-wes