If you are using JMP 16, try making the change interactively and then looking in the enhanced log (View > Log on Windows). Many interactive actions will give sample scripts for the operations that you perform. I tried this with the Blenders.jsl sample data and it gave this:
// Change column info: Time Cycles
Data Table( "Blenders" ):Time Cycles << Data Type(
Numeric,
Format( "Engineering SI", 9 )
) << Set Field Width( 12 );
If you have a dt table variable, you can use that in place of Data Table("Blenders").