I have a single player minecraft world on Xbox one, I would like to play it as a realm, does anyone know how to do this?

First off, to do this, you have to buy Minecraft Realms. If you have already done that, hover over the world you would like to select, and click the pencil icon next to it. Here, you should see all the options. Right next to where it says Play/create, it should say something like "Make a Realm!". Click on that and it should work.