Firstly open the map list and open your Base Map. Now press MAP FOLDER and press MORE MAPS > DIGITAL MAP SHOP > scale of map you wish to re-download.
Then once the little section has downloaded, go to your map folder and press the arrow next to the map type. Then press BULK DOWNLOAD > DOWNLOAD ALL and the system will re-download the mapping for that scale that you previously downloaded and activated.
Repeat this for each scale and type of mapping you previously downloaded.
*NOTE* if you have older Digital Map Shop maps (such as pre 2012 OS map data) you may need to turn the ability to download older maps on first. To do this press MAP FOLDER > MORE MAPS > DIGITAL MAP SHOP > MAP TYPES and turn the entry TOPO OLD to ON