You need Autohotkey for the reassign-script to work. Download here https://autohotkey.com/ 
Documentation here https://autohotkey.com/docs/AutoHotkey.htm

It basically snatches the ~/F1/F2 from dce's clutches and whenever you press one of these keys the one set in the script is send to the game instead.

If you want that Gerald calls his horse when you press F1 for instance, go to Options/Keybindings and assign Numpad1 to "Call Horse".

List of key names you can use in the script: https://autohotkey.com/docs/KeyList.htm

; Reassign Keys - Autohotkey-Script

#NoEnv
SetWorkingDir %A_ScriptDir%


~::Numpad0

F1::Numpad1

F2::Numpad2
