Go to Settings and use Change password. You need your current password. Every other browser that was logged in to your account gets logged out, and this one stays logged in.
In Settings, add your email and type in the 6-digit code we send to it. It's optional, and it's only used for password resets. Once it's verified it can never be changed or removed, so use an email you'll keep.
Press Forgot password? on the login page and type your username. If your account has a verified email, you get a link there to pick a new password. No email on your account? Ask an admin for a reset: on Discord, send a message to Astro. Either way the link works once, for 24 hours. You pick the new password yourself, nobody else ever sees it, and you're logged out everywhere so you can log in fresh.