Hi Soulbird
welcome to the forum!
There are 2 ways to access your avatar to change it...
The first is to move your mouse over your name at the top right of the forum (just above the search box). You will see the following menu
Click on 'Avatar' to access the avatar upload pop-up.
The second is to click on your name at the top right of the forum. This will take you to your personal details page in your settings. At the top you will see your avatar. Simply click on the image to access the avatar upload pop-up.
In either case you will be presented with a pop-up where you can upload your avatar:
Click 'Choose File' then locate and open your desired avatar. Finally click 'Okay' to accept the change and close the pop-up. You can also choose to delete your current avatar and so revert to the ? (if you have selected your gender in your profile but not chosen an avatar, the ? is replaced by a male or female symbol as appropriate).