Minicom

To reset your Cisco 2600 router, just follow the instructions from this link.

If you forgot your router's password (enable password, enable secret, console) you need to reset your router through ROMmon.

To access the ROMmon mode, start your minicom and reboot your router. Upon booting send the CTR+BREAK sequence by pressing CTRL-A F on your keyboard.

System Bootstrap, Version 12.2(8r) [cmong 8r], RELEASE SOFTWARE (fc1)               
Copyright (c) 2003 by cisco Systems, Inc.                                           
C2600 platform with 98304 Kbytes of main memory

program load complete, entry point: 0x80008000, size: 0xacc0f0
Self decompressing the image : ########################
monitor: command "boot" aborted due to user interrupt
rommon 1 > 
rommon 1 > 

Then type this command to change the router's configuration register to 0x2142.

I will assume that you are using GNU/Linux systems (Ubuntu,Debian, CentOS etc.) to configure your CISCO router using CLI.
Open your terminal and install minicom and setserial by typing this command in the terminal.

Check your serial port

The output should be like this:

[   18.439952] console [tty0] enabled
[   20.172975] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[   20.173725] 00:08: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A

set your serial port