How to resize partitions when /system isn't mounted in recovery
To use fdisk without recovery, i.e. while Android is running, you need to root your phone, install a terminal emulator, and install BusyBox (the fdisk command won't be available without BusyBox, which provides a bundle of common Linux tools that aren't included in Android). Note that to install BusyBox, it's not sufficient just to install the app. You need to launch the app and click the Install button in order to install the tools.

The process of rooting is device-dependent, and I don't know specifically how to do it on your device, but you could try Towelroot, which works on a wide variety of Android devices and is incredibly simple to use.