Testing & Editing Theme Without It Being Activate

Hey all – quick question that hopefully someone has an easy answer to. I want to change the theme on my WP page (urbanbedougirl.com) but I want to be able to make the proper changes and edits to the theme before it going live on my blog. How can I do that?

The new theme is going to look very different so I don’t want to put it up until I fix everything. Thanks in advance.