Hello Viktor,
If you mean the account of Adaxes default service administrator that you specified during Adaxes installation, it cannot be simply changed. To change the account, you'll need to uninstall Adaxes, and then install it again using the credentials of the new Adaxes default service administrator.
To preserve your settings during re-installation, you can follow the procedure that is recommended for upgrading Adaxes. For details of the procedure, see section Upgrade to New Version in our Installation Notes: http://www.adaxes.com/resources/InstNotes.htm#upgrade.