Although not always the case, over heating can be the result of overclocking (tuning) This can be done by the user or the vendor from new!
BIOS default settings will restore the motherboard to it's factory "standard" settings. It is this to which you have been directed to check.
You don't say much about your OS or hardware? but to reset your BIOS to default settings you need to enter the BIOS setup during the boot precess and navigate to the RESET TO DEFAULT SETTINGS AND EXIT prompt.
There are other reasons for over heating.
A heatsink clogged with fluff and grunge is typical - or a cooling fan not operating properly - or mismatched components just to mention a few.
Remember though, if you bought your PC already overclocked, by restoring default settings you will lose performance! You really need to know from your supplier to what extent (and just what) was overclocked in the first place, if indeed this is the case.