I had Java Edition installed, and I tried to load up a world on 1.7.10. Every time I would try it, it would crash and say "java.lang.IndexOutOfBoundsException". I went into the settings, clicked "use beta launcher" or something, and then the launcher wouldn't open. So, I tried to reinstall it, but every time I try to do it, it goes through the installer itself, opens the launcher, but when it gets to "updating java runtime" it stops about three quarters of the way through and says "Unable to update the Minecraft Runtime Environment". I've tried uninstalling and reinstalling multiple times to no avail. What can I do to fix this?
Doesn't help you much now but your 'IndexOutOfBoundsException' is a fairly well know issue when you play 1.8+ with render distance above 16 and then go back to 1.7.10 without either installing Optifine or setting the render distance down to 16 beforehand. Probably should have done some research on the issue before you messed up your launcher.