You can define the refractive index of a material to be what you like in CST. If for some reason it stops you from changing it, create a new material with the properties you want. You can also make it a parameter so that you can tune it easily. Relative permittivity = refractive index squared.

Agree with Malcolm White that

refractive index = (μr ∈r )^1/2

μr is the relative permeability equal to μ/μ0

∈r is the relative electric permittivity equal to ∈/∈0

Imadeldin Elmutasim is more accurate. Usually the relative permeability is 1, but not always.