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.
To keep anyone on your network that has the ArcGIS Desktop (and maybe Pro) from being able to use your cache directly (that is, without going thru the service), you would have to restrict access to the cache directory or the ArcGIS Server machine itself. Most users don't even know that they can see the cache directly, so that may not be an issue.
If you are trying to secure your ArcGIS Server, you may want to look thru some of the links below. The dates vary, but since you haven't mentioned a version, you can choose what is appropriate (and they will all have info that is helpful for any version)
I hope that helps get you started.
Rebecca's links are great to get you started.
If you want more specific instructions on this please let me know what your ArcGIS Server version and service pack are as well as the security model of your ArcGIS Server site (that would be ArcGIS Server built-in users and roles or Users and roles from an existing enterprise system -LDAP or Windows Domain).
You can verify this from the ArcGIS Server Manager> Security tab and the Configuration Settings listed there.