You will need to go to guest services- either at the parks, downtown Disney or at your hotel if you are on site. You'll need to do it before you have completely used up all 7 days on your current pass. Then you simply pay the difference and you will get the new pass. If your 7 day expires after 14 days you'll have to upgrade within that time period and you will have to add the nonexpiring option.The cost will still be considerably less than buying 3 new days on a new pass.