If you suspect that someone is using your Gmail account without your authorization, we recommend that you change your password. This way you will protect your information and avoid sending spam or even the theft of your account. For greater security, the new password must consist of letters and numbers.
How To Change Gmail Password
Sign in to Gmail, click the Settings button on the top right corner, and select My Account. In the Sign-in & security section, select Signing in to Google:
Then, click Password. Enter the current password and click Login. Finally, enter the new password, confirm it, and click on Change Password: