The simplest is: http://polaris-bios-editor.eu/. Edit the BIOS with just one click timing patch. You must first memorize the original BIOs and save it if something goes wrong. Then edit the copy of the original BIOS with the specified program. After editing the BIOS, flash the new BIOS with: https://www.techpowerup.com/download/ati-atiflash/.
      I don't think you need 1200 MHz for GPU. For me, 1150 MHz hashes the same as 1200 MHz or higher. A lower frequency will allow you to better undervolt and save more electricity.