You have two different apps for that that works differently, and when I used them I used some apps with one method and others with the other method. I just don't use lots of iPhone apps on the iPad any more so I hack the apps manually.
Fullforce will stretch the apps to fit the whole screen, some apps get graphical artifacts, or look horrible. Others looks so good you wonder why the developer did not include iPad support using only iPhone files.
Retinapad will trick your apps to use higher resolution artwork and higher resolution fonts in 2x mode. Nearly all apps look very good with this method but you get a non-intrusive black border around the apps (I hate intrusiveness, but this is not bad). Apps can look somewhat blocky in 1x format, but it does not matter since its not very noticeable and you nearly never use apps in 1x (at least not me)
Both apps are paid tweaks.
