How do I replace a ledger board?

you need an expert.

That looks like something a professional should do, to me.

Yes - you need a professional as you are dealing with the foundation of your house and do not want it to shift or settle.

Call a professional structural contractor, this is not really a diy project.

House needs to be lifted, old board removed, New board set, then lower house......also looks like you might need a termite inspection, or a termite company who knows right kind of contractor for you to call.

my husband did this in the front of our house. I think it is work for a skilled carpenter. He replaced part of the sill, and siding, and it was a significant but not impossible job.

I hope it's not around the whole house. Dont try to fix it yourself 😨 At least get a professional to look at it. The house might sag or shift, and cracks will be made anywhere in your house. You need a professional to lift it. Think of your drywall, roofing, windows, plumbing! You might still get a crack but at least they will do the best job.

Prise it off with a Claw hammer. Looks as if you have a Damp Problem! Make sure when you replace it, you use Marine timber as it will hold up better.

I agree with everyone. You need a profession contractor for the repair. The house will need to be jacked up, old wood removed and replaced, house lowered. Not really a DIY job.