Hello,
The account of Adaxes default service administrator that is used to run Adaxes service cannot be changed as simple as that. To change the default administrator account, you need to reinstall Adaxes. When installing Adaxes anew, you need to specify the new Adaxes default service administrator.
When reinstalling, we recommend that you follow the procedures specified in section Upgrade to New Version of our Installation Notes (with the only exception that yo can use the same version of Adaxes as you have right now).