Can I straighten this stand without taking it apart?

First check what’s actually out of square. Set the stand on a flat surface and compare the two diagonal measurements across the rectangular frame. If they differ, gently clamp the longer diagonal toward square and recheck before adding anything. If the frame also rocks on the flat surface, it may be twisted rather than just racked out of square.

If the glue has already set and the joints resist, don’t keep cranking the clamps. You could split a joint or distort a board. A diagonal brace or corner gussets can help hold the frame square, but they won’t fix boards that are themselves badly bowed or twisted.

A triangle will make the stand much less likely to rack again. If you can bring the frame square with gentle clamping, fit a diagonal brace across the open side, or add gussets at the corners, then fasten them while the frame is held in position. Check that the brace doesn’t interfere with how the table will be used.

If it takes a lot of force to get the frame into line, I’d stop there. With the joints already glued, cutting and rebuilding the worst connection may be more reliable than trying to force the whole assembly straight.

Also check the pieces themselves. Soft construction lumber can move as it’s cut or as it dries, so a frame can be out because one rail is warped, not because the assembly just needs a shove. A brace can stabilize the shape, but it won’t turn a crooked rail into a straight one.