Occasionally, some of us need to reset the AEM Admin password of a local copy of a CRX quickstart image, either because we did not keep careful notes of old versions of the admin password or never had it. This occurred recently when we received a production publish backup from a client to check our new code against.
If you are lucky enough to be still running older versions of Adobe Experience Manager (6.0, 5.6, or 5.5), the Help instructions from Adobe are great (https://helpx.adobe.com/experience-manager/kb/reset-admin-password.html or http://experience-aem.blogspot.in/2015/07/aem-6-sp2-reset-administrator-password-to-admin-offline.html). The admin user profile was stored under /home/users/admin/.content, and you could put a package with that JCR Node from another AEM instance with a “known” password into the crx-quickstart/install before starting up that instance.
But if you are running AEM 6.1 or newer, the admin user profile location is more secure as a “guide” location somewhere in /home/users (http://labs.6dglobal.com/blog/2015-06-03/changing-user-passwords-aem-61-curl/). In the screenshot below it shows under c, but that is just where ours happened to be because that is what the system set the first character as. Your results may vary.
The fine folks at Shine Solutions Group have a newer solution (https://github.com/shinesolutions/aem-password-reset) which can be installed at startup, and worked great for my AEM 6.2 SP1 problem. I built the package via maven, put it into the crx-quickstart/install before startup, and was able to log in as the default admin/admin to then change it to something else I could remember.