Change your password
Tap your name at the bottom of the sidebar, then Settings. On a phone, tap the menu button (≡) at the top right first.
In Password, enter your Current password and a New password of at least 8 characters.
Tap Change password.
You stay signed in on this device, and every other device is signed out.
Set a password on a Google account
If you signed up with Google, Password says "You sign in with Google. Add a password to also sign in with your email." Enter a New password and tap Set password. You can then sign in with Google or with your email and password.
Forgot your current password?
Sign out, then tap Forgot password? on the sign-in page.