Accessing data in a ByteBuffer to edit it

Hi,

How do I get the data from a ByteBuffer so I manipulate it? (And then put it back into the same ByteBuffer)

Reason being, I want to flip an image vertically when I load it, that way I have less inconsistencies in regards to UV co-ordinates, from my post process frame buffer, UI and all the loaded models.
Right now, I have to flip the UVs vertically on the loaded models, but not on my frame buffer, apparently OpenGL flips the image if it’s put in normally, I have no idea why.