Two Configuration register mode:

0x2102- Normal mode
0x2142- Bypass mode

Steps:

1. Restart your Router
2. Press Ctrl+Alt+Pause/Break
3. Type “confreg 0x2142”
4. Type “reset”
5. Now you can access your router in bypass mode
6. Now copy startup config to running config
7. Set your desire password
8. Type “config-register 0x2102”
9. Copy running config to startup config

Thats all Now Enjoy.

Here is the Video Tutorial.