How can I edit SpecificGravity?

In parts there is a property called SpecificGravity, it there a way to edit it?

According to the Wiki:

This is a read-only property. It cannot be written to, only read.

So, no.

You simply have to pick the material with the specific gravity closest to what you need.

This property only affects the buoyancy of objects in Terrain's water, which is at the least a small detail to not be able to control finely.