You will need a valid password sooner or later, so you may as well recover that first. Basically, almost any changes you will have to make, or any new software you want to install will require a password.
You could try following the directions for what to do if the password is lost, here:
It is also helpful to know what is actually installed, if for no other reason then b/c people here will ask what version you have.
If you can open a terminal window in your guest login, then try:
or, for more information:
this is what mine looks like:
peter@Flattop:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 14.04.2 LTS
peter@Flattop:~$ lsb_release -r