Does your current form use the built in Form block? If so, sign into the dashboard, go to the page where the form exists, click EDIT on the top left and then click on the Form you wish to edit and in the dropdown select Edit. It's pretty self explanatory from there.

If you have the HTML form all ready to go, simply create a new HTML block and paste your code when the dialog box shows up and it should work.

Check out the first section on this page:http://www.concrete5.org/documentation/...

I have tried that and the box for the choice of forms allows me to save my 'contact form' and still not edit.

Please could you try to explain again?

Also in the documentation I don't know which section i am supposed to be looking at. I will also need help with adding landing pages after this but the box that comes up for adding the page isn't there. Is there a possibility that my access to all of the features is limited?

Did someone create your theme for you? If so, it's possible that they have some limited permissions for you. Honestly I would start at the beginning of the documentation and watch as much as you can. It's going to make working in the CMS much easier for you.