Look carefully at the base portion of the zerk (where it meets the hub).
If the perimeter of the zerk is "hex shaped", it is a threaded fitting.
If it is round without any "tool flats" present, it is a press in (aka "drive in") type.
Removing "press in" zerks can be done by using a sturdy pair of vise grip pliers and a large, flat blade screwdriver (or similar suitable prying tool).
Get a "good bite" on the zerk with the vise grips, then place the tip of the screwdriver between the pliers and the surface the zerk is seated in.
Hold the pliers steady and try to pry the pliers and zerk straight outward.
Twisting or rocking the zerk might "wallow out" the hole in the hub which could cause the replacement zerk to fit poorly and simply pop out when you apply grease pressure, or when you try to disengage the grease gun tip.
Press in zerks CAN BE replaced with standard, threaded zerks.
The main issue when trying to replace a press type with a threaded type is that there is no readily available tap that cuts threads for your typical zerk.
For the times I have had to "convert" a zerk, I use a 1/4" coarse thread tap to just barely make some threads in the existing zerk hole. Then insert the threaded zerk and keep it upright and square as you turn it clockwise. Threaded zerks are hard enough to "cut their own threads" in softer metals. It can be trial and error until you get the feel for making your starter threads with the 1/4" tap.
Try to get out any cuttings from the tapping before you apply grease through the new zerk.

There is an available "drive in" tool for installing the press in type zerks but in nearly all the cases I have encountered problems with press in zerks.........the hole itself had been damaged (impact damage to zerk from foreign object) and had to be converted to thread type.
I have made drivers for the press in/drive in type by drilling a hole in the tip end of a grade 2 or 5 bolt that is just big enough diameter to let the grease end of the zerk enter, but the perimeter of hole rest on the base circle of the zerk.
Then you can strike the bolt head with a hammer to seat the zerk.
Good luck!

Good follow-up Mownie! I managed to fix a factory miscut Zerk thread by carefully removing all grease from the thread by using Qtips and carb cleaner. When you think all gease is removed, clean it two more times. I then applied a small amount of JB weld and screwed the zerk in finger tight and let it set over night before use. So far the repair job has lasted 4 or 5 years.

Thanks guys, it had hex shapes sides and screwed out and was replaced.

Thanks for the feedback.