The free and the paid version of FreeCell are separate applications. You can have both of them installed on your device at the same time, the paid does not overlap the free version when you installed.

Check your device for another install of FreeCell that has the same icon as the free version, this will be the paid version with no advertising. You are still playing the free version if you are getting ads.

If you can not find it, go to the App Store icon on the main screen of the device, tap updates, tap purchased, tap not on this iPhone box and search for FreeCell in the list and reinstall it.

Just note that the statistics are not transferred over as Apple treats every application independently, so you will start fresh with the paid version.