How do I change my Gmail password, or reset it if I've completely forgotten it?

Two different paths. To CHANGE it (you're logged in): go to myaccount.google.com > Security > 'How you sign in to Google' > Password, enter the current one, then the new one. To RESET a forgotten one: on the login screen click 'Forgot password?' and Google walks you through recovery, it'll try your recovery email or phone, a prompt on a signed-in device, or the last password you remember. Having a recovery phone and email set up beforehand is what makes this painless. Turn on 2-step verification while you're in there, it's the single biggest thing for keeping the account safe.

The 'Forgot password?' link on the sign-in page is the whole reset flow. It leans on your recovery email or phone, so if those are out of date you can get stuck. Worth checking them now before you're ever locked out.

Change the password AND turn on 2-step verification. A strong password on its own isn't enough anymore, the second factor is what actually stops account takeovers.