Short answer: A B450 board probably won’t need an update.
Long answer: It depends. A BIOS update IS required to support the 3000 series. When a motherboard is made, they put the latest available BIOS on the board at the time. That means if the board is old, aka was made before the 3000 series came out and was sitting in a warehouse for that entire period before being delivered to you, it would need a BIOS update. If it was made after the 3000 series came out then chances are it came with a BIOS that supports it. Some motherboards openly state ‘3000 series ready’ (such as MSI’s ‘MAX’ series of B450 boards) meaning it does come with a new BIOS to support that series. The problem is you can’t really know the BIOS version the board has till you get it. Some boards also have a sticker indicating the installed BIOS version on the BIOS flash chip as well.
As to how to update, the motherboard manual indicates how you can go about it. Read that. If you’re speaking about how to update the BIOS when you have an unsupported CPU, you have several options:
Borrow a supported 2000 series CPU. You can do this via a request with AMD, where you fill out a form and they send you a temporary CPU. After you update it, you ship it back to them. Optionally if you have friends with a supported CPU, you can save time with the process. Some people even carried the board in to hardware stores and got them to update the board there.
If your motherboard supports BIOS flashback, you can update the BIOS without needing a CPU installed. This will be indicated in the motherboard manual. Not all boards support BIOS flashback either.
Some users reported unsupported CPUs working in boards that state it wouldn’t be. This isn’t something you can bank on as it probably is very unlikely. If the system does boot then you can update the BIOS from within the BIOS menu.