service engine soon light on

the light came on about two weeks ago. i have a 2001 maxima ae with 36300 miles i know the 3 year/36,000 miles is over i wanna get the extended warranty. But i have to get this fixed first i did a diagnostic the codes are P0140&P0142 downstream 02 sensors bank1 & 2 the place where i took it said he would charge me $450.00 to fixes it what should i do get them myself and change them is it easy to do or let him do it or go to the dealer please somebody help thanks

I would just do it myself. It's not that hard of a replacement. The sensors are kinda expensive though. Shop around and you might find a good price.