Welcome to the forums, newcomer!
Assuming you have a bootable USB drive hooked to one of the USB ports on your system(or motherboard), follow through this;
https://www.asrock.com/support/faq.asp?id=27
if you hit the option for Boot Menu, you should see the bootable USB drive show up as one of the options, selecting it and hitting enter will make the system boot to the USB drive.
Here's another example;
View: https://youtu.be/CyepX0DHeTs
You could also choose one of the drives listed in Boot Override to entre into a bootable environment(provided said drive has one to begin with).
Hope that helps.