How can i update regup-sgtxt field in sap script

Hi experts,

I have created sap script for payment print prog. In that one field is amount discription "REGUP-SGTXT". i want to use field "REGUP-SGTXT" as a discription. for sap script payment print i use T-Code f-43, fb03,f-58... now in whic T-code i fiiled the value of "REGUP-SGTXT"?? how can i update value in my sap script?? cause i ceateed so many doc no but "REGUP-SGTXT" its blank... so how can i fill this fiels???

Give me suggetion

Zenithi