How to charge previous (When my app was free) user after make it paid?

My iphone app was free for long term. I ve made it paid any more . But i want to charge people who already owns it if they want to update, i dont want to allow them to get updates unless they pay. I ve seen some apps are asking for paid uptades but i dont know how to do that, any idea ? 

How to charge previous (When my app was free) user after make it paid?
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My iphone app was free for long term. I ve made it paid any more . But i want to charge people who already owns it if they want to update, i dont want to allow them to get updates unless they pay. I ve seen some apps are asking for paid uptades but i dont know how to do that, any idea ? 
