What to do with post with dry rot?
Are you sure about the load bearing? It may sag if you are not careful...

Anyway either replace the post completely or make a replacement bottom section and support the rest of the structure - acrow jacks work well for this type of thing.

This is one example of a supplier of acrow jacks - many others... :
https://www.scaffolding-direct.co.uk/new-size-3-acrow-prop-2-4m-4m/