I think you have a funky number of sides to be able to do this, which is why I tend to work in multiples of 8 or 12 for cylinders, but: * I delete half of the cap, so I'm left with a semicircle's worth of tris. * Then I do a "Hinge From Edge" on the remaining tris, with the open edge as my hinge, by 180 (set to however…