I want to change the username and password for afaq shamel modem. do i have to restart the modem after changing username and password. moreover, do i have to keep the modem connected with my laptop while changing username and password

You should not have to restart your modem after changing the password. Once you change that password you might be promt to enter your new password in to establish an internet connection following the password change. The preferred method to to access your modem is through your computer with the modems IP address.By default most of the afaq shamels passwords are as follows:
Default IP: 192.168.1.1
Username: Afaq_shamel
Password: stccpe_2007

Type in that IP address in any internet browser (Fire Fox, IE, etc) You should be prompt to enter in a user name and password. If the one mentioned above does not work try:
Username: admin
Password: password

I hope that this helps you along the way. let me know if there is anything else I can do to assist you.
Thank you