sql server 2012: cannot alter the login sa

A little more specific :

1. Open Sql Configuration Manager.
2. Select SQL Server Services.
3. On the right hand side, select the instance.
4. Right click on it and open properties.
5. In the advanced tab attach ";-m" at the end of the Startup Parameters field.
6. Apply and restart the service.
7. Now you have privilege to enable "sa" user and modify its password.
8. once done, remove ";-m" and restart the service.
9. You are good to go.

I found the answer here:

In order to start SQL Server in single-user mode, you can add the parameter “-m” at the command line. You can also use the SQL Server Configuration Manager tool, which provides proper controls for the file access and other privileges. To use the Configuration Manager tool to recover your system, use the following steps:

1. Open the Configuration Manager tool from the "SQL Server 2005| Configuration" menu
2. Stop the SQL Server Instance you need to recover
3. Navigate to the “Advanced” tab, and in the Properties text box add “;–m” to the end of the list in the “Startup parameters” option
4. Click the “OK” button and restart the SQL Server Instance

I'd like to point out an alternative answer laid out on DBA SE. Download PSExec onto the box that is having the problem and follow the instructions laid out in this blog post to effortlessly change admin settings using the NT Authority\System account.

./psexec -s -i "C:\...\Ssms.exe"

Wanted to share this solution as it solved my problem!