You do not need to use the Add a new scenario page to update existing scenarios you have uploaded, you must access them through the My Scenarios link on the "Index" Page
What is the 'My Scenarios' link for?
This link is a powerful tool that all designers need to know how to use properly, the page lists all of your uploaded scenarios in the testing area and allows you to change almost all aspects of those scenarios. The link is only active while you have scenarios uploaded in the testing area.
The page is self explanatory, but here are some tips
You cannot upload and replace an existing scenario file while any tests are registered against that scenario.
You can prevent any new tests being registered against your scenario by changing the 'Testing In Progress' box to 'Testing Stopped', you will need to do this when either you have enough tests running or you are waiting for current test to finish so you can modify or approve the scenario. If you modify the scenario remember to change this box back to 'Testing in progress' to allow tests on the new version to be registered.
When your scenario has the required score a link will appear in the 'Approve' box to allow you to move it to the Approved list.