After the release of the dark below DLC Xur has sold upgrades to exotics that were out before hand. IF you have any exotic items that you acquired before the update then you can purchase upgrades for them from Xur (random selection each weekend). But don't buy the upgrade if you got the item recently. It will still offer you the upgrade for them, but if the stats on the upgrade screen are the same as the ones already on the item then you don't need to upgrade it.