It can be done in the vehicle but it is tight. Depending on the engine a 4cyl has a timing belt and a 6 cyl has a timing chain. Since you will have to remove one I would do a timing set at the same time but is not required. You will have to make a punch mark on the pulley so it is in the same position. I found one on a Volvo you could move so definately do this most only go on one way but I'm not sure on this one. Once the pulley is off use a screwdriver and remove the seal. Use a big socket for a seal driver.
Just take your time and be careful. Buy a Haynes manual so you install the timing set properly.