1Gather the Components
You'll need thread sealant tape for installing the new gauge. As a maker/hacker, it's likely you already have some but it's good to double-check before taking your filter apart.
You'll also need a new pressure gauge. The pressure gauge you need is one that is back-mounted 1/4-inch threaded fitting. Often you can even find these as marketed for using in pool filters.
I used the one from the link below:
This is a Poolmaster 36672 Pressure Gauge.
2Remove Housing Assembly from Filter
This step contains the first important item/trick you need to know. To remove the pressure gauge housing from the tank adaptor you need to unscrew the connector. However, the connector may turn in the opposite convention than you would think. It is not screwed down to the top of the tank but rather screwed up to the housing (you'll see once you get it apart). Here's the important part, turn the connector clockwise when looking down at the top of the filter. A strap wrench or channel locks will work well. You'll have to hold the housing to keep it from rotating.
3Remove Front Bezel
This is the second important item/trick you need to know. It's also the reason we removed the housing from the filter. The pry slot to remove the front bezel is on the bottom of the housing. It's very difficult to reach while the housing is attached to the filter. Once removed, however, it's easy to access.
Use a flat tip screw driver or a similar device to pry the front bezel off. Start at the pry slot. Once you pop it at the pry slot, it should give you enough access to pry at other locations if needed.
4Remove the Pressure Gauge
The pressure gauge unscrews from the housing. It unscrews in the manner you'd expect, counter-clockwise when looking at the face of the gauge. You should be able to unscrew it by hand.
5Install the New Pressure Gauge
Wrap some pipe thread sealant around the threads of the new pressure gauge. You can then screw it into the housing.
6Reinstall The Housing
Snap the front bezel back on then you can reinstall the housing on the tank.