The diagram shows the bearing below the bottom plate which means you may have to press it off. Replacing bearings isn't that hard if you've done it before but there are a few things you need to know about regarding how to do it. For example you can sit the bottom plate across the open jaws of a vice and put a board on the end of the armature and tap the board with a hammer and it will usually part company with the bearing after a short while. Someone needs to be holding onto the armature to keep it from falling. Also, when you install he new bearing you cannot put any force on any part of the bearing except the outer race.
One of the most difficult parts can be installing the armature and getting it past the brushes. Without being able to see the router I can't say how hard it will be. If all of this isn't clear then you may want to have someone do it who has done it before. It isn't tremendously difficult but it helps to see it done at least once. There may be some you tube videos on it.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.