How to downgrade my Java 8 to Java 7?

I'd suggest uninstalling and reinstalling it.

You need to get this from the Archive section of the Java Downloads at oracle.com:
http://www.oracle.com/...39210.html

If you're running a 32-bit Windows OS, I suggest getting the "Offline" version. It seems to handle unusual situations better than the online ("install from the web") version does. It always works when either one of them will work, so it's a "heads I win, tails I don't lose" choice.

You can install this without uninstalling Java 8. If Minecraft can't find Java 7 when both are installed, then you can:
(a) uninstall both versions, and reinstall Java 7 only (not recommended), or
(b) say goodbye to Minecraft and find a new addiction.

If you have to choose option (a), you should disable the Java plugin in your browser. (Some would choose to do that, anyway.) There have been too many holes patched since JRE 7 went off life support.