This used to be possible with the old menu, with the new menu update you cannot rejoin a world you are already in. A popup comes up saying you are not allowed to join an instance you are already in. Why? This shouldn't happen, reloading the world is commonplace in VRChat whenever something goes wrong, and with the current beta it is no longer allowed.

EDIT: There seems to be a canny for this already made here https://feedback.vrchat.com/open-beta/p/unable-to-rejoin-current-world-through-main-menu