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