@Sunel
Hi,
If you don't have the password to connect to the router via Wi-Fi, then you will need to connect a computer to the TL-WR940N via Ethernet cable (yellow LAN ports) to be able to access the configuration interface via http://tplinkwifi.net or http://192.168.0.1
To log into the router's configuration interface try the username admin and password admin. If you get the message wrong username or password, then you will need to reset the router to factory default settings by pressing and holding the little "Reset" button.
It is also possible that your TL-WR940N already has the newer type of firmware that doesn't have a username. In that case you should be asked to set and confirm a new password of your own choice.
Once logged in go to the menu "Working Mode", select "Range Extender" and then click [ Save ]. Then continue to configure the TL-WR940N as you wish.