The default password for administrator user admin is admin. If the password was changed to something else, to reset it to admin without logging in...
Thanks to Yuval Ararat for the tip
1) Stop AEM 6 instance
2) Place the package reset_admin_pass_to_admin.zip in <AEM_Home>/crx-quickstart/install (create it, if there isn't one)
3) Start AEM 6 instance
4) Package should have installed. It contains /home/users/a/admin/.content.xml with rep:password set to text admin encrypted
5) Login as admin/admin