how to clear the field value

HI,

I have a scenario: i have two drop down fields A1 and A2 fields, A1 field contains A and B values, when i select A value second field A2 dyanmically comes into enable.

when i select any value of the drop down list of A2 should be populate in Web Ui screen. again I am selecting A1 field value B dynamically A2 field comes to disable mode.and remove the data what should populating in previous........

I done coding up to enable and disable the the A2 field.

I want to remove populating data on A2 field.

how resolve this.