Official Mango ROM for HTC Mozart has been leaked and it has been made available by the kind folks at xda-devs (link). Earlier, DFT released Windows Phone 7.5 ROMs for HTC’s windows phones but those were not the official ones. This one is official, that will be released in Europe in two weeks from now.
Folks at windows phone review have written an awesome tutorial on how to get and install the latest and official windows phone 7.5 / mango. Basically, you extract the EXE file you downloaded earlier and remove the SPL_signed.nbh from the list of files. Then, execute ROMUpdateUtility to flash the ROM. Read more at windows phone review