Power off the machine, then power it on again and press repeatedly the appropriate key several times while the computer is starting

Settings ⇒ General ⇒ Boot Sequence: USB Storage Device must be selected, and in the first place of the Boot Order

• Dell:
  • Healthy Windows computer: check the support page
  • Crashed Windows, or healthy (or crashed) Linux computer: use a USB key to update the BIOS, following the steps in the How to recover from booting issues using the BIOS Recovery Tool video
• HP:
  • HP Consumer Desktop PCs - Updating the BIOS (Basic Input Output System)
  • HP Consumer Notebook PCs - Updating the BIOS (Basic Input Output System)

There are lots of HP BIOS details in HP PC Commercial BIOS (UEFI) Setup - Administration Guide (pdf document)

Read the Windows Recovery Environment (Windows RE) and the Safe Mode notes

Power off the machine, then power it and press the appropriate key at the same time

See also Performing a safe clean boot

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