You can change the password by loading YunSerialTerminal over USB and using passwd
root@Arduino:/# passwd
Changing password for root
New password:
Bad password: too weak
Retype password:
Password for root changed by root
root@Arduino:/#
you can ignore the too weak warning it isn't enforced
The Yun is getting it's IP from the DHCP server on your router if it keeps getting 192.168.1.168 it is because your router keeps giving it.
I have seen threads of people with strange problems which were traced to "non-standard" behavior of their router, can you try using another router?