Hi Tarun.
As of now you cannot change Purple Line(BYPL to Mysore Road via Majestic) to Green Line(Mantri square to Nagasandra)- once green line is extended till Majestic then interchange point u'd be Majestic.
If you still wanna do this then take Auto from Mantri square to Majestic Metro and then change the metro(s). Btw from where to where are you planning to travel by metro?