With your website you have the power to add, edit and remove new administrator users as you like.
Login to your website
Search for Users
Add / Edit / Delete Users as you see fit
NB: Adding user > Make sure to select "Administrator" in the "Groups" area when creating a new user so they can login to your website & manage it via the CMS. See screenshot below.
Forgotten password / reset password
Go to your login page (e.g. https://yourwebsite.com/login)
Go to the "Forgotten Password" area
Type in your email
Click button to reset
Check your email, and especially your spam / junk mail. You should receive an email with a link via which you can reset your password
Follow this link & reset password
Login as normal