When you go to your profile by clicking on your user name in the top right corner (if you are on desktop), you should be able to change your profile picture by clicking on the image and then selecting an image you want.
if you are on the mobile app,
then get to your profile by tapping the icon three lines on the top left and then click on your name.
This will bring you to your ΓÇ£account detailsΓÇ¥ page. From there, scroll down until you reach avatar.After that click the image (or in this case it will be your letter) to select a new image.
If you want to use your YouTube channel icon as your avatar then you can find this image in the promotion section of your TubeBuddy account (not the promotion section on the forum).
You can find it by going to your Dashboard on TubeBuddy. Then click promo tools. This will take you to a whole page worth of links and images from your YouTube channel that you can use to promote your channel. This includes a link and image of your channel icon.