This question is identical to this one: How do I grant myself admin access to a local SQL Server instance?, except... my question is about SQL-Server 2012, not 2008. The answers from above question do not work, since, as it seems, I do not have enough rights.
The problem: I have a SQL-server instance (SQL-Server Express 2012), the original owner is killed (the account is killed, not the person himself), so no one is sysadmin anymore.
I am local administrator on the machine and want to grant myself the sysadmin-role on SQL-Server, without reinstalling it.
How?