Welcome to the forums!
I wouldn't really recommend switching to WEP because it can be cracked by just gathering enough packets of data. WPA2+PSK is the safest way to secure a network but if you want still want to change it then goto 192.168.2.1 which should be your routers local IP if not, then check its manual for information about that.
It will prompt you to login, use the following if you haven't changed the password:
Username : admin
Password : MiniAP
then click on Security in the Wireless section of the task pane on the left
then in security mode, choose 64 bit WEP
Enter a password and save
it should work
Good Luck! and let me know if it worked