Unfortunately you won't be able to completely "uncrush" the tube again.
Remember that we are dealing with cardboard here, which is essentially a mass of compacted fibers. When you create a crease (or two as in your photo), you are effectively destroying the fibers' structure and tearing some of them. Straightening the crease with mean you still have torn fibers at the former crease, no matter which method you choose.
If you want to reshape nevertheless, pressure on the two creased ends alone will create a parabolic shape and likely that new crease in the middle (or at a weak point somewhere) and the "square" result. At minimum, you need to "hold back" the center with your second hand, molding the original shape. Combined with a cylindrical balloon (blow up once first and deflate again for easier use) from Willeke's answer, you should be fine.
But there's a big caveat:
If you need tubes with maximum stability for some "load bearing" in your project, the reshaped ones might look good, but will bend again at the creases almost immediately. You might want to discard them and resort to intact ones.