There is a few ways of doing it (practice them all)
You can mirror across the x axis (only) “copy pose” from the frame number you want to mirror and then “Paste mirrored” on the frame you want to mirror. This will not work for the vertical axis – only the horizontal axis.

You can duplicate the keyframe with exactly the same values (in the action editor or GE) and then type a “-” on the approriate channel. (or remove the “-” if the value is negative)

Or you can also do some fancy mirroring in the GE (Graph Editor).
Check out this menu

You will most likely want to mirror it “over Value = 0”.
Practice, make mistakes, learn, and get better 🙂