When you open a Skillz game for the first time, you are automatically signed in with a random username, and occasionally your email address will not be linked. To change/update your account information, go to the main menu in the bottom right corner, tap Account, and then Personal Info.
Review your information, and edit your personal details accordingly. To update your email, you may tap the Edit button beside your current email address. This is also the same screen you'll verify your email address on. When you are done, tap Update at the bottom of the screen to save your changes; you may need to scroll down.