It depends on the tool you're using.  The generic method is just erasing the tags from the building and creating a new node and adding the deleted data.  There are some other tools that can automate the process - in iD you can click the building, rightclick and select "extract" (a little dot with an arrow pointing away from it) and it will automatically make a POI node that contains the business data within it, and also deletes the data from the building itself.  LMK if you need more details or if you want someone to do this.