Go to https://accounts.google.com/
Click your profile picture in the top-right
Select Sign out
If prompted, confirm Sign out
To sign out on a specific device/browser:
Open the Google app or browser
Go to your account/profile menu
Choose Sign out
To sign out from all devices:
Go to https://myaccount.google.com/
Select Security
Under Your devices, select Manage all devices
Choose Sign out on the device(s) you want or select Sign out everywhere
On Android/iPhone:
Open Settings
Tap Google
Select your account
Tap Remove account or Sign out (wording may vary by device/OS)