If you signed up with an email and password, you can change that password any time from Settings. We'll require your current password as a safety check.
Open Settings → Authentication.
Tap Update Password.
Enter your current password.
Enter and confirm your new password.
Signed up with just your phone?
Passwordless (phone) accounts don't have a password, so there's nothing to change here. You log in with a one-time code instead. If you'd like a password too, open Settings → Authentication and tap Set password.