I tend to have my Substance and the .sbs project file named the same. It's not necessary, but it's less confusing to me. You can have multiple Substance graphs in a single file, in which case they cannot all have the same name as the file that contains them. But most of my projects are the simple, single-graph type of project.
a) Right-click to SaveAs to a different file
b) Right-click to change the name of the Substance (a.k.a "Material")
That should do the trick.