How do you replace a main seal on a 2000 toyota solara?

Rear main seals mean that the transmission has to come out. Allow for 8 hours for the job.
An accredited service centre will be able to do it in less so get quotes first

Why do you want to know? How many miles on the car? Does the Main seal front or rear actually leak? If so I would get a second or third opinion. What is the history of this engine's seals? There is both front and rear main seals. Front if it's leaking is much easier to accomplish. As Bill says, I recommend you not attempt to do it yourself.