Minecraft – Why can’t I change the username

Having played on a cracked version before you need to completely uninstall your Minecraft, because more often than not cracked Minecraft versions typically use a proxy to bypass the login phase of the download. So long as you have the old install, your Minecraft will still use the proxy to authenticate. Additionally, a lot of those proxies pair with your actual Minecraft account, so that could present some issues after you paid.

I'd suggest clearing out your .minecraft folder and deleting your minecraft.exe, download a new one from the Minecraft website, reinstall your mods (according to the instructions given by the modder), and you should be good to go.

Make sure that when uninstalling, you don't delete these folders:

• saves
• texturepacks
• stats
• screenshots

These folders all store data (Such as your worlds, achievements, and screenshots.)

The launcher has 2 flavors. A java based installer and a non-java based installer.

Both version install the launcher. Once that is done the launcher and the game do require Java. Given your post I would recommend that you download Java 8 for x86-64 if you do not already have it.

When you download the launcher installer be certain to ONLY download it from minecraft.net after logging into your account. If you download it from any other site or attempt to use a 'cracked' version then I wouldn't expect it to work and may Notch have mercy on your soul.

As with the game, be certain that you download Java Runtime ONLY from the trusted site.

If you have errors with Java 8 x86-64 then I would begrudgingly suggest uninstalling and installing Java 7 x86-64 instead.