Is there an easy way to rotate a texture without completely remaking the texture?

I am making a mod that adds a sword like weapon to the game but because the texture goes from the top left to bottom right instead of bottom left to top right, the player holds it wrong. I think just flipping the texture would work but I don't know how to just flip the texture instead of completely remaking it from scratch, which I'd rather not do. Is that possible to do?