1, you can choose the startup item of ESC, and generally choose the startup item of U disk that doesn't start with UEFI;
2, BIOS, boot press F2 to enter the BIOS- boot items-option # 1 select the first boot item.
If it still doesn't work, please tell me the specific model of the machine and help you check whether you need other BIOS settings.
I hope the above information can help you! If the above reply still doesn't help you, you can continue to ask questions or contact ASUS service hotline 400,600-6655 for consultation! thank you