easiest/proper way to repair rotted pine roof wood

People,

I have a pitched shingle roof, has years left to it. But, there is a spot that the nail on the shingle was exposed, and previous owner neglected. As a result, wood piece below it rotted over a year or 2, befor I bought the place.

So, Were talking about only say, a 4" wide pine board, and from rafter to rafter, say, 14-156". Thats it. To replace that pine board, do I have to yank a few shingles above it, then cut out the pine, replace, then shingle back? Or can I do this from underneath somehow, without disturbing the shingles?

Thanks,