Where you're signed in
See every browser signed in to your account and sign out the ones you don't recognize.
Find this under Where you're signed in on your account page.
The list #
Each row is one browser signed in to your account: the browser and device, when it was last active, the country, and when it signed in. The one you're using is marked this device.
Sign out a device #
Click Sign out next to it and confirm. Anyone using it needs a new sign-in link to get back in.
When more than one other device is listed, Sign out all other devices signs out everything except the browser you're using.
If you don't recognize one #
Sign it out. There's no password to change, so signing it out is the fix. Then check that every address under Emails is yours.