1 Sign out of your Apple ID
To download Pokemon Go you need to create a new Apple Id. Make sure you sign out of first of your existing Id. You can do that through the store or the settings section on the iPhone.
2 Change your region
Go to settings > General > Language and region then change your country to one of the supported countries like USA, for example.
3 Create a new Apple ID
Create a new Apple id by visiting Apple's site. Make sure the ID belongs to the same region you chose earlier.
4 Verify your email address
Apple requires that you verify your address after creating the ID so that you can use the ID.
5 Sign in with your new ID
Go to the Apple store and sign in with your new ID. You should see the apps available for that region you created the account for.
6 In case you don't find it
If you don't find it this means that the old store was loaded instead. You will just have to find the link for the game online through your browser then click on it. The link will be opened through the Apple Store and it will ask you whether you want to change the store or not.