1. To imports add: import XMonad.Actions.SwapWorkspaces
2. Somewhere in file, where you have keyboard mapping add:
-- Swap Workspaces , ((modm .|. controlMask, xK_Right), swapTo Next) , ((modm .|. controlMask, xK_Left), swapTo Prev)
3. Reload xmonad, now you can swap workspaces using combination Mod+Ctrl+Left and Mod+Ctrl+Right