Extend visa duration.

Dear Sir, I am a widowed retired professor. My daughter is working for ABB over 20 years. Last year I obtained a "Visiting Relative" visa for 365 days (Type D; multiple entrance; valid before 30-07-13) and stayed in Vesteras since August 2012. However, for some personal reason I had to be back to Beijing at beginning of November and am expecting to continue my visit to Sweden in late May 2013. I strongly wish to extend my duration of staying in Sweden after 29-07-13 for another three months. In this case, what should I do? Looking forward to your earliest reply. Thank you!