This is one of the techniques to mass change data in NAV.

1. Create a new report, selecting the table you want to fix.
2. Set the ProcessingOnly property as shown below

 

 

CAUTION:  Make sure you set a filter on the dataitem to prevent accidental changes. 

// set a hard filter to prevent accidental changes
setrange(County,'TX');

*** TAKE A BACK UP OF THE TABLE in case you mess it up. Better still test in in a test environment, then run in your live environment***

Then write some code in the OnAfterGetRecord trigger

 

if "Payment Terms Code" = 'NET14' then
  "Payment Terms Code" := 'NET30';
modify;