It may work now, but as soon as you push that card in an intensive game, your computer may crash or even have a brownout. Worst case, your PSU would go up in smoke.
Changing a PSU is really easy, just unscrew the 4 screws on the rear of the case holding the PSU. The cables for the mobo and such are self explanatory, put in new PSU, then put 4 screws back, it would take you at most 30 minutes to do this. The best way to learn is hands on experience