The records received by Cumulus from the station are kept in the various xxx.ini files in your \data folder. Unlike Cumulus 1, in CumulusMX you will need to edit that erroneous data manually in those files (with CumulusMX stopped).
You may also want to edit the Jul19log.txt log file, as well as dayfile.txt if the bad data shows up there after the midnight rollover.
You might consider setting the Spike Removal in the Calibration Settings to eliminate unreasonable data coming from the station.
Enjoy,
Paul