asus m5a99x evo , when i go to the asus support site it only take me to the rev 2 mobo support page if i isntall that bios will it bsod my pc?
If you attempt to flash your BIOS with a different version, one of two things could happen: It may not even let you complete the update or it may brick your motherboard. It is not advisable to flash your board with anything other than the BIOS intended for your make/model and revision.
What is the model and revision of the board you are asking about? Why do you want to update the BIOS? Updating the BIOS is not something that should be done on a regular basis unless you are experiencing issues that are corrected with the updated firmware.