how can i display the textbox data dynamically?

Hi,

i am having form name like vatform in that one textbox,one button is there.issue is if user wants to update the vat then he dont no what is the present vat.for that i want to display the present vat in that textbox and it was display in runtime if he click on that it will erase and he will update the vat value how can i do this?

Best,
Sudheep.