They don't have an instruction manual online at the Black and Decker support site, but if you want a look at the exploded diagram, it's
here.
Basically there's a collet nut that gets tightened down to hold the bit in place. You've got to loosen it up and you'll be able to pop bits in and out. You've got to have some way to lock the shaft while you're doing this though.
Most modern routers have a spring loaded shaft lock button. While you're holding it in, start turning the collet. When the shaft gets to the point where there's a little hole for the spring lock, you'll probably hear a little click, and the shaft will be locked so you can loosen the collet. If you take your finger off the shaft lock, it pops right back out again so hold it down while you're loosening and tightening.
If you don't see a shaft lock button, you may have a second hex nut at the base and with two wrenches, you can hold the hex nut and turn the collet loose that way.
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