@Faye alright you are using Raspbian/ Debian Jessie. Jessie is the older version as everyone stated. You want to Stretch and that will mean you need to start from scratch with new image. That is no problem at all and good way to learn :) .

BEFORE you start with fresh image i'd highly suggest you copy to your computer the roms, bios, configs folders connecting through your computers network. Now you said wifi isn't connected?

This video below by ETA Prime will walk you through the whole setup of a fresh install of RetroPie. But skip to 6:13 mark in the video there he shows how to setup wifi on the Pi.

Setup your wifi when done back out to the selection screen click '' show my ip '' write down the ip address. And now go to your computer and go to file explorer. Find and click network and give it few secs to show up '' RETROPIE '' .

Now if you don't see RETROPIE when you click network this is where the ip address comes into play. At 7:31 of the video where he puts RETROPIE in you replace that with your ip address you grabbed from the Pi.

Click Retropie and now copy to your computer roms, bios, configs folders.

Once you have copied the folders i asked you to copy from the Retropie image you are using now.

You will now have to get erase the RetroPie image you have now in order to install the new RetroPie image. Follow this video on how to do so.

After you have erased your micro sd card you will need to download a image here https://retropie.org.uk/download/.

Now watch that first video from start to finish and everything should be good.

Sorry if this makes no sense. I'm just trying to help.

Ropi 4.3 - Intellivision doesnt work