Welcome to the forum Angie. If you check the manual, it shows on page 3 figure 1 how you change bits. In the schematic on page 6 it is parts 55 and 56 that are required to lock the armature in place to be able to change bits. I saw a date of 1979 on one item in my google search for info. Parts are likely unavailable but someone who is fairly handy could improvise. If it is missing those parts or the small tang that fits into the router shaft is worn so that it won't stay locked then it is close to worthless as far as retail sales go.
I know Sears used to have regular sales on various router models and may still. You might be better off with a new one. At least you'll be able to get parts. If you do go that way google the model # and add reviews to see what others have to say about it. Some of their routers have gotten really bad reviews and some were pretty good.
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.