rpc password is set in bitcoin.conf:
find / -name bitcoin.conf
Highlight the full path and file name, hit Ctrl+Shift+C
Type
nano, hit Space and then Ctrl+Shift+V
This should look something like this
nano /home/user1/.bitcoin/bitcoin.conf
Enter
Might be nano is not installed, so you could use vi. nano is easier to work with for most though, so install it with
sudo apt-get install nano
Related reading material:
https://en.bitcoin.it/wiki/Running_bitcoind#Bitcoin.conf_Configuration_File