You need to set the value of the dataView not the textbox. While working with epicor it is highly recommended you do not use controls to drive your program. Rather use the data and the dataviews as drivers.
You may want to do some cursory reading of the Epicor Customization Guide before you jump in with both feet so that you start out doing things the “right” way as they say.
In your case you’ll want to get a hold of the appropriate dataView (as shown below) and set the value of the field
var myDv = oTrans.Factory("NameOfYourDataView");
myDv.dataView[myDv.Row].BeginEdit();
myDv.dataView[myDv.Row]["FieldName"]= destFile;
myDv.dataView[myDv.Row].EndEdit();
Also you may want to rename your controls from button1 and epitextboxc8 to something that makes more sense, when you come back to this customization 6 months from now you’ll have no idea what any of this means if you don’t take the time now to rename those fields.
Sorry for all the extra advice but it looks like you are starting at the beginning and it is way better to learn this stuff now.
Again please read the Customization Guide, the ICE Tool Guide and the Programmers Guide available in EpicWeb before you do much more development, you and your company will be much better off.
PS: Also read our FAQs and learn how to format the code in the posts it’ll make things a lot easier to read.
Thanks!