How do I round edges of a terrain?

Questions & Answers
1

I am trying to make an island but I don’t want it to be a square island, help? I at least just need to round the corners of the terrain a little.

2

Select your terrain. Go to the inspector and you can see 3 icons like a mountains. With that icons you can paint mountains and modify your terrain.
The oficial documentation of unity has a lots of basic information: