Came back to my apartment to discover that a careless subtenant had misplaced my only postbox key.
Front view:
https://imgur.com/a/SeImhQ7
https://i.imgur.com/9b0kzGL.jpeg

Inside view (I managed to [miraculously] open my lock with a neighbour's key. It won't lock back though). The black stuff is the heavy tape I'm using to hold the door shut.
https://imgur.com/a/tcwfwJZ
https://i.imgur.com/lZ8wdjH.jpg

If it matters, the building is a 1960's Berlin apartment block. From the looks of my box and others, I doubt they've been replaced since.
Does anybody know how I would go about replacing this lock? Is this something that a total beginner could do, or should I call the local locksmith?
I'm grateful for any help here. Thank you in advance
