As far as how to change the resistor, my suggestion is actually to not change the resistor if you've ever worked with metal, have a drill press, or a corded drill. Even better if you have taps. If you haven't worked with metal, it may be a good idea to learn (But if you don't have a corded drill or a drill press, and can't borrow one, just go ahead and change the resistor. I'll leave some instructions at the bottom of this post).
I suggest upgrading to a ceramic heater cartridge and a threaded thermistor (You could also just add a threaded hole for a screw to retain the thermistor, Ala E3D).
To do this you will need (Links are just a convenience, you can source equivalents elsewhere):
A 12V heater cartridge, 8.99$ USD
http://www.amazon.com/12v-40w-Heater-Ca ... B00I50FUTI
An M3 threaded thermistor, 11.99$ USD
http://www.amazon.com/Modular-Thermisto ... 00THZJIY8/
An M3 grub screw, 2.86$ USD
http://www.amazon.com/Bolt-Base-Stainle ... B00B3RIS3A
An M3 Tap set: 7.79$ USD
http://www.amazon.com/Metric-Flutes-Bot ... B00VGEG5GM
A 2.5MM drill bit
Use the drill bit to drill a hole through one side of the aluminum by the resistor, through to the cavity, being careful to not go further. Then proceed to use the taper tap in the hole to produce a threaded hole. (In theory, you can just keep spinning, but I prefer to go two turns forward and half a turn back for ease, and to ensure long tap life, especially when working without tapping fluid) Go all the way through the hole, and make sure your grub screw fits all the way in. If it does, that's good (It should screw in, not fall through, mind you. Had someone do that for me once). It will retain the heater cartridge in that side of the hotend. Don't insert it now, since you have a little more work to do.
Drill out the thermistor hole carefully, so as to not change the depth. Once that's done, use the bottoming tap to tap the hole to accept the thermistor. Then you can go ahead and screw the thermistor in, insert the cartridge, and tighten the grub screw (You may need to ream/drill out the resistor hole if the cartridge is to large.) Then change the temperature sensor setting in the firmware to match the sensor type (There's a list in the firmware, or you can google the right one for repetier), and retune the PID. You are now done.
Otherwise:
Buy two new resistors, butt connectors, and likely thermistor from SeemeCNC. Find/Buy Permatex Ultra copper RTV and kapton tape elsewhere, alongside solder and iron if you don't have it. Strip old RTV out of hotend. Try to keep old thermistor in one piece, and in the hotend. Fail utterly. Coat resistor bodies in RTV. Insert into old holes in hotend, do same with new thermistor. Despair at the mess you've made and that it's not nearly as neat as the factory job. solder old leads onto new thermistor, and then bend together and crimp the leads on the resistors to the power leads going to the hotend. (One lead per resistor to each wire, please). Wrap the hole mess in Kapton tape, and hope it holds together for a while. Reinstall in 24 hours, and resume printing.