The Bootloader uses the first 2 pages (0 and 1) of Flash memory and the application starts from the 3rd page of Flash memory. Therefore, to erase first page of application the Erase Page Memory command (0x80 0x05 0x00) should be called with a parameter "0x02" as shown below:
0x80 0x05 0x00 0x02 - to erase first page of application.
Similarly to erase the second page of the application, use the command :
0x80 0x05 0x00 0x03 - to erase second page of application.