Take the two nuts off the back. They hold the ammeter on as well as make the connections to the circuit.
However, I've ruined two gauges because the studs turned when I turned the nuts, and that broke an internal wire. So instead of using a nutdriver I now use a wrench and hold the end of the stud with pliers the first time I take one off.
Gary, AKA "Gary fellow":
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI