I'm not sure I totally understand you question, but I'll give a few answers.
if you are asking how a user, with a connection to a service in ArcMap/ArcCatalog might reconnect after they changed their password.....you can right-click on the connect file and change the password stored there. This should only affect secured services. I tend to not store the password with connections just because we have to change our password often.
If you have lost access to services because your user store, i.e. The user that gives you access and permissions within AGS manager has changed its password, you can go back into the security in manager and update it. For this, it is recommended to have a system account that has a complicated, non-expiring password.
if you admin (or is it service?) account was changed and isnt working so you need to reset, there is a script to help with that.
It may help if you can give a few more details on what you are trying to do, and maybe a graphic snapshot of your error. Otherwise it might be hard to give anymore advice.
