Assuming it is functional, just pour out the old and add new. There is a screw top with the hose that goes to the tank on the reservoir on the back side of the gauge. There will be small metal rods in there too; do not remove they are for calibration. Lots of threads on this, search "hydrostatic" and they will come up.
https://thirdgenauto.com/product/193...l-gauge-fluid/