You haven't given enough detail in your question but I will have a go at answering what I think is your question.
"Now i have updated membership type to 1 year expiration".
I am taking that at face value, ie your membership type called "our membership" did have a duration of 2 years but has been updated so that it now has a duration of 1 year. (If instead you have created a new membership type of one year duration then what follows is irrelevant to your situation.)
If these are paid memberships, then I would expect that the existing expiry dates should stand but when members renew they should only get an 12 month extension to their membership end date. That would be normal CiviCRM behaviour, so I have ot assume that is not your situation.
That suggests that these are free memberships and you want alter the expiry date of the existing memberships. That seems a little out of the ordinary. I am not sure I would be happy if an organisation said to me "Yes, we know we said your membership expires on 31 December 2017, but we changed our minds and now it expires on 31 December 2016".
Still, if that is what you want to do, you need to realise that the end date of a membership is fixed when the membership is created, so you will have to change the existing end dates.
I would do it by exporting the membership data, altering the end dates in a spreadsheet then importing the membership data with the new end dates.
When you export the membership data I suggest you export membership Id, membership type, membership status, member since date (or is it "membership join date"), membership start date and membership end date. The importer always needs more fields than I think it should, so it is better to be get all those fields out in just one export.