Amplifiers contain voltages high enough to Kill you even after the amp has been powered Off !!! If you don't have enough electronic experience to identify and discharge the Filter Capacitors which contain these Deadly voltages, take your amp to a Qualified tech. If you don't know a qualified tech in your area, post a request in the alt.guitar.amps newsgroup, the folks there will be happy to help you.
1. Note the position of each lead and remove old wire. (Put it in a plastic bag, and put it away in your closet for preservation).
2. Purchase a new 8' to 10' heavy duty 3-wire (grounded) power cable and a plastic strain relief strap from any good amp parts supplier.
3. Strip away about 6" of the outer insulation and strip about 3/4" off the end of each wire.
4. Solder a round lug to each of the 3 wires.
5. Attach the green wire(ground) to the nearest transformer bolt you can find.
6. Attach the black(hot) and white(neutral) wires to the same positions where you removed the original black & white wires from.
*NOTE* If your 2 prong power cord amp has a Ground Switch, you must also disable this switch for Safe operation of your amp. For instructions, see the "Ground Switch Mod"