Darling, if you have saved the new av on your desktop, here are the next steps:
1. click on your old av on the upper right hand corner of the normal playing screen
2. on the little window that popps up you'll see Change Avatar and right next to it there is a grey Browse button, click on that
3. another window should pop up where on the right hand side chose Desktop and it will show all the files you have on your desktop, chose the one you've saved the new av under and at the bottom of same window click on Open and that should also close that window
4. back in the Preferences window click on the Upload button and after that on the Ok button on the lower right hand corner of same window
5. close the window and hit F5 to refresh
And that's all there is to it. Good luck!