If for some reason you need to open "top menu" again
There is "star" icon on toolbar to open top menu (same one that opens on client launch (if "skip menu & load default scene" option is disabled).
-however as TToby siad that doesn't load new scene and your current scene is still loaded (behind it)
-usually when i wanna switch stuff i just open new empty scene (true, it doesn't unload stuff)
-i have default scene replaced with empty scene (also checked "on startup skip top menu and load default scene")=placebo effect for faster startup
-there are unload options (for skins & unused items on person), also "clear cache" (not only for textures)
-even with those there is "memory leak" and after 5-6 scene loads (with lots of stuff) things are still left inside ram (from previous scenes)
if you have less than 16gb ram client could crash more often (not that 32gb makes much difference, but for anything bellow 16gb try unloading stuff to reduce crashes or client restart (if you are loading multiple scenes)
also if you are working on something it's best to restart client and load only that scene (nothing before it)