Uboot does not have relationship with your firmware. So upgrading your firmware does not affect uboot.

to use uboot web failsafe, the key is to obey the sequence:

  1. Plug only one cable to the router from your windows. Do not plug 2 cables to the router
  2. Hold down the reset button
  3. Power on the router, wait and see one LED flashing, until another LED light up
  4. Release your finger.
  5. Set your static IP in your windows (you can do this first) and open your browser.

Check out the videos linked from here