Can't change DNS on S3 mini
First enable USB debugging.
You could use adb and use the command root or su shell and copy the build.prop file to /storage/sdcard/0/ or /storage/exsdcard with the copy /system/build.prop /storage/sdcard/0/ command (path of your (ex)SD Card storage vary from device to device). In this file the DNS server IP is saved. Find it and change it to the OpenDNS IP. Now copy it back to your /system/ folder with copy /storage/sdcard/0/build.prop /system/. Restart your phone and it should work.