Timber Retaining Wall - Beam Replacement
It might not be that simple. Many wood retaining walls use nails or rebar to lock each tier with the one below. You will likely have to remove them from the top-down, but you probably need to anyway. If that one is rotten all the way to the exterior, the others are likely almost as rotten.

I recommend taking it apart completely, removing what needs replaced, and staining/sealing everything (new repairs and old beams) with a good exterior sealant, and building it back up.