We are replacing an overhead microwave, the refurbished came with no instructions,
How do we install, and what do we need to buy at the home depot first?

What brand is the refurbished one? You can probably get some instructions from the manufacturers website. You'll have to install the wall bracket and attach the MW it to the upper cabinet, they usually come with a template that is very helpful. I bet the manufacturer will send you one if you call them.