You can access your profile page by clicking on your avatar at the top right, or hovering over it and clicking "Profile" on the navigation bar. It should look something like this:
On this page you can edit your Salad username, select an Avatar, and enter your game usernames for applicable rewards.
You can edit your username by clicking the box, entering your new username and clicking the submit button:
You can select a new avatar by selecting it from the list shown. You can earn new avatars by taking part in Salad Seasons. Your game usernames can be edited the same way as your Salad username.