A BIOS update is no trivial thing. … You should also update your BIOS if there are critical security flaws that need patching or you intend to upgrade to a new CPU. CPUs that are released after your BIOS was created may not work unless you are running the latest version of the BIOS.

Should I update BIOS before installing new CPU?

Yes you will most likely need a bios update. If you don’t and put the new cpu in it probably won’t post.

Do I need to update BIOS after installing Windows 10?

A System Bios update is required before upgrading to the this version of Windows 10.