HWiNFO UI -> gear icon -> custom tab -> RTSS -> Framerate -> Unit section (top right).
In the past/prior updates I could rename this to whatever I wanted (e.g. change FPS to F or / or whatever) -- I would just type it in under the Unit heading and hit Rename.
After the latest update this no longer works for some unknown reason and it always resets itself back to "FPS" no matter what I rename it to. I never have had this issue before, not sure what's up.