I am afraid there is no manual and even if there was, that wouldn't help much. You would need some understanding of relational databases and SQL to be able to modify the NetWorx's database - after all it's not supposed to be edited by user.
Assuming you are already using something to open the database file, run this statement after changing it as required. It will add 300GB of incoming traffic to user John Smith dated Jan 30, 2017:
INSERT OR REPLACE INTO usage (USER_NAME, DAY, HOUR, DATA_IN, DATA_OUT, DIALUP_DURATION)
VALUES ('John Smith', cast(julianday('2017-01-30')-julianday('1899-12-30') as integer), 0, 322122547200, 0, 0);