1. Sign in with your Office 365 admin account at https://portal.office.com/adminportal/home.
2. In the Admin centre, go to User>Active user.
3. Select the one user to reset the password.
4. After selecting the user, Click the Reset Password icon.
5. Click on the Let me create the password option.
6. If don't want to change the password when first sign in, then uncheck the Make this user change their password when first sign in option.
7. Create a strong password, then click on the Reset button.
8. Password reset done.