Open a browser and visit:
Login with your router user name and password. The default user name is root and the default password is 'none'.
Go to System > Backup / Flash Firmware
Under Backup / Restore section click the Generate Archive button next to Download Backup
Open the created tar.gz file in archive manager or similar decompression program.
Within the archive go to /etc/openvpn/key.txt
Open the key.txt file. The user name is on the first line and the password is on the 2nd line.
If you want to change the user name and password just modify this and save. You will need to accept the changes in the archive manager or otherwise extract these files and tar them back up. Then perform reset and then upload the tar.gz file using the restore backup choose file button and Upload archive... button.