auto populate field from drop down list

Hello, I created a drop-down list and a text field, I automatically fill in according to the list but sometimes I need to modify the text in the text field, I can't;
for example the ZCQJ006 can be 70 and can be 80 and sometimes even 85, please help

drop-down = NGAPCCAM       text field1 = paiement   text field2 = Somme

javascript = 

var f = this.getField("NGAPCCAM");

var g = this.getField("paiement");

if ((f.valueAsString=="ZCQJ006") && (g.valueAsString == "CMU")) {this.getField("Somme").value = "70";