Look on Google for Xperifirm, there you will download the firmware you want, then follow
this link to create a ftf file with the files you download, and this
other link to learn how to flash it to your phone.
FYI, first thing I did when I got my D6603 was to flash the Custom US ROM, but after a week I decided to use the Custom UK (BTW it's the only region with latest .105 version) and besides that it doesn't show the LTE icon (4G in UK is LTE) the rest is the same, but I must be honest it seems a little bit snappier and faster, IMHO, so I'm keeping this one.