OK here we go with the tutorial....
1, Install Root Too from here... 
https://www.windowsphone.com/en-us/store/app/root-tool/f4acc009-ea1e-4063-9bab-fec50195aa1e
2, Move it to SD card.
3, Put the SD card in the memory card reader and go to WPsystem folder
4,Delete the following two files from the installation path.. 
sdcard\wpsystem\apps\f4acc009-ea1e-4063-9bab-fec50195aa1e> install
      mdilfilelist.xml and wmappmanifest.xml
5, Move all the files under folder HACK to installation path.. 
sdcard\wpsystem\apps\f4acc009-ea1e-4063-9bab-fec50195aa1e> install
6, Move the app back to phone memory.
7, Run the Root Tool app
8, Open
 Lumia Registry Editor
9, Selcet 
HKEY_LOCAL_MACHINE
10, Select 
DWORD
9, In the path type 
System\maps\storage
10, In the Key type 
UseExtendedStorage
11, Press 
Read
12, it will show 
00000000 in the value field
13, Change the value to
 00000001
14, Press 
Write
15, In the 
Key field type 
UseSmallerCache
16, Press
 Read ensure it reads as 00000000 if not change it to 00000000 and press
 Write
17, In the
 Key field type
 ExternalStorageOverride
18, Press 
Read.  ensure it reads as 00000001 if not change it to 00000001 and press 
Write
19, Go tot the maps and download a new map.
20, Go to Storage Sense and select SD Card
21, Maps session should show the maps size you have just downloaded.
I thank
 djamol from XDA forum for the wonderful
 Root Tool.