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.
Ask an admin for a password reset: on Discord, send a message to Astro. You'll get a link that works once, for 24 hours. Open it and pick a new password yourself. Nobody else ever sees it, and you're logged out everywhere, so you can log in fresh with the new one.