I cannot seem to unscrew the water filter head from the sump

In my new home came installed a MT660 filter and I am trying to install a new ceramic candle (MT661 RFC-Ultracarb).

I cannot seem to unscrew the head from the sump, as I feel I am about to break the unit. Any ideas? Tips or tricks?

1. Turn off the water to the filter

2. Open (turn on) the filter faucet, this will relive the back pressure

Once you change the Ultracarb candle apply a bit of vegetable oil to the sump "O" ring before closing.