How to change the d-link model dsl-2640t wireless password

Which password? The admin password to get into your configuration screen?
If you have lost that you can reset the unit. Check your manual for steps to do that.

Press and hold the tiny reset button on the back of the unit. Hold it in for at least 10 seconds then release. Next connect to the unit using a CAT5 or better cable and connect in your browser to 192.168.1.1. The name is 'admin' and 'password' or 'admin'. Addtionally, try admin and no password if the former doesn't work.

Try admin and admin before resetting all programming to default. If it is not 192.168.1.1, we can help.

How to setting up d link DI 524UP Wireless router password