Enter the BIOS menu of your motherboard. To do this, immediately after starting the computer (before OS load) then click DELETE (in most cases). For some motherboards you will have to press another key (e.g. F1) or combinations thereof. In any case, when the first lines of the loading on the monitor there's always a message stating what key to get into BIOS menu.
If you have a motherboard with AWARD BIOS (most common), look for Advanced BIOS Features. In it you will see the items First boot device, second boot device and third boot device corresponding to the first, second and third source download.
In AMI BIOS used in ASUS motherboards) go to BOOT, where you will see a Boot Device Priority where you can set the order of download sources.
If your motherboard BIOS is different from what is described, then the description of working with it certainly is in the instructions to the motherboard. In any case, the General principle is slightly different from that described above.