As any other Android application, you can set it in so called manifest, descriptor file containing all meta data and info how android can communicate with application (which is very simple in case of UE4).
You can find manifest of your application in Intermediate/Android/APK/
Also it important to understand that Android APIs version (and as result Android version) in manifest as well in any android code is described by API levels. Each time Google do changes in API new API level is created, for example Android 4.0 is level 14, 4.0.1 also level 14, but 4.0.3 is level 15 because been API changed in this version, you can find table translating those numbers here:
Keep in mind that changing min version wont magically make UE4 on older systems, you should keep it as it is which is level 9 (android 2.4) which reaches most of android user base, so this is ok