dm_setvarint NW_JOURNAL_ENTRYnameofquesthere #ofquest
so if the quest tag was "find_the_stinky_shorts" and the number you wanted to update to was 25 it would be
dm_setvarint NW_JOURNAL_ENTRYfind_the_stinky_shorts 25
Remember that you have to have the PC selected for this to work