There are two ways to accomplish your goal here. Option 1:
If you look at the top right corner of the page, you will see your username. If you click there, a menu will appear.
Click on Account Details, or here is a shortcut https://forums.magicalguides.com/account/account-details
This might be easiest on a mobile device.
Option 2:
You can click the right-corner, and open that menu again, if you roll your mouse over the "M" in your case, you will see the edit button appear. Click on that and it will launch the upload avatar option.