How do I convert my working branch from HEAD to master?

[adrian]

Hello. I am fairly new to SourceTree and GitHub. At a point in my project I had to go back a couple steps to a working commit. I did a checkout on that commit and did my rework from there. Several commits later I realized my now working commits are labeled HEAD. 

I would like my commit at the top to be my master commit, how do I go about doing that?

Here is a screenshot.

Thanks