The Main IAS Administrator is a super user which always has full permissions (Level 10) on all the applications and is allowed to configure the system. This "first user" is created during setup and cannot be deleted.
The Main Administrator can create other users with extended permissions, so you can have an unlimited number of administrators allowed to manage some or all the applications.
The Main Administrator is not included in the IAS users list. It cannot be associated to a Voip phone, telephone number, fax number, personal email and so on. His purpose is only to perform the configuration of the services. For this reason, this account has no access to the Attendant Console, cannot send faxes, use click to dial, have personal directories...
If you give administrative privileges to a IAS user, he or she will not have such limitations.
From top-right "ADMIN" link, available in any web portal's page, the IAS administrator can can change own username and password.
If you did forget the main administrator password, you can reset it by using one of below options:
IAS version < 2020.Winter.1
IAS version >= 2020.Winter.1