Here's full turorial https://www.youtube.com/watch?v=vUEMS-B1Siw&t=791s
Usually when overclocking GPU without 6pin its limited to 75W and safe temperaturs are around 70-85*C (safer under 85*C).
You can damage your card by only increasing Core Voltage of GPU too far, but thats not possible i think in your situation.
Here's full turorial https://www.youtube.com/watch?v=vUEMS-B1Siw&t=791s
Usually when overclocking GPU without 6pin its limited to 75W and safe temperaturs are around 70-85*C (safer under 85*C).
You can damage your card by only increasing Core Voltage of GPU too far, but thats not possible i think in your situation.