I need help on how to replace bit on router craftsman style

You didn't state the model number. The older Craftsmen routers had a red lever lock on the top of the router, one position allows the spindle to turn, the other position locks the spindle. Here's the instruction for the older style routers:

1. Unplug the tool.
2. To lock the spindle press the red button in and slide it to the locked position.
3. Unlock the height adjustment knob.
4. Raise the router motor using the height adjustment collar until you can get a wrench onto the collet nut.
5. Loosen the nut until it turns freely.
6. The bit may still be locked in the spindle, tap it gently (not on the blade!).
7. The bit will come loose in the spindle and can be removed.
8. Replace the bit, tighten the nut snug but don't use excessive force.
9. Unlock the spindle, returning the red lever/button to the free position.
10. Adjust the height of the bit as you wish, then lock down the motor using the locking knob.