Super simple.
Log into your account.
If you're already logged into your coaching account, navigate to your profile icon in the bottom of the left side bar
Click into Settings
Once in Settings, the General tab will be opened automatically and you'll see your coach profile menu.
You can then add or change your coach avatar in the avatar icon beside your profile name
This will then automatically apply in all the accounts you own or have been invited to join in your 'My Accounts' listing.
Thanks
FITR