Are the threads galled or are they stripped? If they are galled and you just want to remove the parts that are rolled over, do as walove says and get an M6 finishing tap and just retap it. Use light machine oil.
If they are stripped, you may be SOL. You might be able use an M6 helicoil kit. These are just threaded inserts made from diamond cross-sectioned machine grade steel drawn wire wrapped into a coil to fit universal 60 degree thread patterns. It's basically an insert with its own internal threads that fits into another threaded insert where you want to restore the original threads. Tap out the insert per the installation instructions and insert the coil with the tool and you have new M6 60 degree threads. The problem is, you have to evaluate whether or not you have enough material on original threaded insert to tap out and hold the new helicoil. These things are incredibly strong, though and if there is enough meat left to retap, it will work.