First we need to create a new Self Signed Certificate for the existing web application.
Click on the root node in IIS 7 Manager and select the “Server Certificates” icon in the feature pane on the right:
Click the “Create Self-Signed Certificate…” link on the right side of the window.
Enter a name and click OK. Now you should see your certificate which appears in the list of server certificates.
Select the IIS site that is running the SharePoint application and click on the “Bindings…” link on the right hand side of the page
To enable SSL click “Add”, select “https” and select the SSL certificate we created earlier:
Click OK. At this point we have configured IIS to allow connections to this site over SSL but we need to let SharePoint know to map these requests to the correct web application.
Configure SharePoint to accept requests over HTTPS
Navigate to Central Administration > Operations, select “Alternate Access Mappings” and click “Edit Public URL’s”
Select the Alternate Access Mapping Collection for the web application and enter the relevant HTTPS address. It is up to you which Zone you put the URL in as this doesn’t affect anything and is just there to help you remember what they are each used for.
That’s it! You should now be able to browse to your site using SSL. If you are using a self signed certificate you will first be presented with a certificate warning so click on the “Continue to this website (not recommended)” link and your site will be displayed.
I found this solution here.