Hello,
You will need to first turn off the program and then follow these steps:
go to:
C:\ProgramData\Graitec\Advance Steel\2013\Shared\Support\Prototypes
create a new folder in there and call it "Imperial prototypes"
now move everything that does not have mm_ in front of them into that new folder
note that the mm_ represents the metric prototypes
so by moving all the other ones into the new folder, the system will only read the metrics.