How difficult is it to replace the left rear wheel bearing assembly and what's the part that I need. It it hums when I'm driving 20 to 40 miles an hour. Toyota mechanic said that it was the bearing

It is well beyond the average DIY'ers capabilities! .. Special tools and equipment required as well as talent to do the job... Hydraulic shop press .. Metal cut off tool or acetylene torch and the ability to use use the torch 'surgically' just for starters!

About $450 with a qualified mechanic.