Step-by-step guide
Let's say you want your message to use the product definition template 40 (The full list is defined in Code table 4.0):
% grib_set -s productDefinitionTemplateNumber=40 input.grib2 output.grib2
Now you can do a "grib_dump -O" to inspect the section 4 (Product definition section) of the output.grib2 file to see the new key "constituentType" (For other templates you will see other keys added).