This is a general Unity question. Please have a look at how to tweak shadow settings on the official Unity documentation pages.
If you mean the shadow is too "jaggy": Here it looks as if you are using hard shadows and the shadow map does not provide the resolution that you would like. You could switch from hard to soft shadows to hide the problem. In general, shadow map resolution of each shadow cascade is not as high as you would expect it to be, as it has to span e.g. 1024x1024 shadow texels over a large area. The only hint we can provide here is to tweak the shadow settings as described here.
