Random variables transformation?

Questions
1

Hi,

I am trying to transform a PDF function of x to a PDF function of r using Mathematica, noting that r=x^2 and the PDF of x is as below:

f[x_] := (( (4 Sqrt[?]) (?^(? + 
         0.5)) (h^(?)) (x^(2 ?)))/((Gamma[?]) (H^(? - 
         0.5)) ((?)^(? + 
         0.5)))) (E^((-2 (?) (h) (x^2))/(?))) * 
  BesselI[? - 0.5, 2 h x^2 ?/?]

Anyone can help of how could I do this?

BR,