You can get a crossbow bowstringer. I got mine from Excalibur along with an extra string after I got my Ibex. You may also be able to pick one up on Ebay.
All you do is put the stringer on the limbs, cock the bow using the stringer which puts slack on the string. Remove the old string and reinstall the new one (through the stinger holes) and slowly uncock the bow being sure the new string is in the retaining groves on the limb tips.