The panels I've purchased recently have not needed squaring, but in the past when I bought some pillow panels that weren't square I was able to square them by washing the fabric and pressing it the shape I wanted. Fabric is wound on the bolt damp so if it gets slightly askew, it dries that way. Washing it will relax the fibers.
If that doesn't work, then try Mike's way. Put a frame around the panel that matches the outside border and square the 'new' block.