where is the apple wallet app ?

The easiest way to find it is to do a spotlight search on the phone. Swipe the homescreen to the right and you will see search. Type in Wallet, and it will locate it. If it is in a folder, it will identify the name of the folder to the right of Wallet in the search results. If you just loaded the iOS and did not move it yourself, then I believe it appears in a folder. The title may be Utilities, or something like that.

Another way to find it ... if you sync with iTunes you can find it there by "sort by name" in the apps list for that device .