Follow the instructions to remove the drive belt and then loosen the bolt to remove the idler pulley. The bolt is a long one and you can hold it in place from the other end to remove the bolt. Installation is just the reverse.
I assume you mean the idler pulley below the lower green arrow in the picture. Hopefully you will see the nut on the pulley side so you can pull the long bolt out a bit to get the pulley off. If you see the head of the bolt you will not be able to get the pulley off without sawing off the head of the bolt and then pushing it through the other way. Some came from the factory with the bolt installed the wrong way or if you got a rebuilt motor the bolt could easily be installed backasswards. I think it was MarkG who won the bolt in backwards sweepstakes on his factory motor.