I have an oil painting,and I want to change the frame,but it has glue on it. How do i remove the glue?

If it is an older painting it might be held on by hide glue which can be taken apart with heat, try a hair dryer from behind and see how the glue responds to the heat. Then again from behind see if a little flap of the painting is behind the frame, see what the heat does to it. You might find the exact right amount of heat to melt the glue but do no damage to the painting.