There are settings for the leader key in your
.spacemacs file. I have remapped mine to Page Up, (which is represented as
<prior> by emacs). Here's what I have in my
;; The leader key
;; The key used for Emacs commands (M-x) (after pressing on the leader key).
;; (default "SPC")
;; The key used for Vim Ex commands (default ":")
;; The leader key accessible in `emacs state' and `insert state'
;; (default "M-m")
You may not need to set all three here as I have, start with just the leader key accessible in emacs state ("M-m") and see if that works.