How to replace Manjaro with Linux Mint?

Hi, I have Manjaro in a desktop computer but want to switch to Linux Mint. How can I do that? Thanks!

That would be a complete reinstall as they are differeing based systems(Manjaro - Arch, Mint - Debian/Ubuntu)

Backup your data externally to USB drive or thumbs etc.
Download Mint ISO, and create a bootable USB as you did to install Manjaro.
When your ready, boot the new Mint USB and install.
When the installs complete, begin customizing.

You are correct. Hopefully they set up a separate partition or drive for data when installing Manjaro as it will be easy to wipe the os part then install fresh.

OK, I already have the bootable usb with linux mint in it. How can I access the BIOS in Manjaro to set up the boot from the usb? I tried with escape, f2, etc while rebooting but it didn’t work. Thank you!

That would depend on the brand/model specifically. It may be “DEL”, or “Enter”, “ESC”, “F2”, “F10”, “F12”… This varies by manufacturer.
There is usually a quick splash that gives you the option when you first power on, some give a second splash specifically for a boot menu which includes booting form a USB device.