There is no standard easy way to scale a texture by a given factor. It depends on what you are trying to achieve. If you want to have the texture being displayed larger, you would just use either different texture co-ordinates on the primitive the texture should be mapped on, or use another modelview transformation or projection.
If you really want to build a (let's say) 512x512 texture out of a 256x256 one, you would need to scale the texels yourself in the host program and create a new texture from it with the new size.
Then, to scale the texture yourself, you would need some sort of filter (not just nearest neighbour) so that the result looks good.
Another (faster) way to scale a texture would be to render a fullscreen quad with the original texture mapped to it into another FBO that has a texture with the new bigger size attached to it (as well as an appropriate viewport with the new size being active).
That way, you use the hardware texture filtering (which is LINEAR by default).
The FBO with the bigger attached texture can even be the same as the one you created the original smaller texture with. You just need to create a new texture of the bigger size and attach that to the FBO and then render the fullscreen quad with the original smaller texture.