Your account
Change your email or password, and see where you're signed in.
Everything about your account is under Settings › Account.
Change it in Settings › Email. We send a link to the new address, and the change takes effect once you confirm it.
Password
Change it in Settings › Password. Forgot it? See I forgot my password.
Devices
Settings › Sessions lists every device signed in to your account. Revoke any you don't recognise, or sign out everywhere else with Revoke all other sessions.
Signing out removes your data from that device. If it was offline, check Settings › Sync first, so nothing that's still waiting is lost.