I don't exactly know if one can do that but it's better you don't. When you take the SIM holder out there is a code there like 059J233 etc and Navifirm has firmware listed for each code for each country and by now I guess it has for all the variants. If you have bought in US itself then you can look within US list & then this kinda code and the file will be the one with PR_LEGACY in it.