The tampered thing is nothing to worry about, it just means that the bootloader has been unlocked.
You might want to look into getting S-OFF by following one of the guides here on xda.
But it sounds to me like you are in the bootloader, you should be able to use the volume buttons to select recovery to enter TWRP or CWM.. in recovery mode you can flash the software update if it's a ROM that you found here as a zip file.. There are many guides around to do all of this, good luck.
Edit: Press volume down twice to select bootloader then hit the power button. Then you should see another menu with an option for recovery. Are you rooted? Do you have a custom recovery installed? If not, I'd recommend downloading TWRP.