If you want to use an existing survey when you are creating a new survey you can click on 
on the same row as the survey you want to copy. In the opened pane you enter the name of your new survey.
Now you have created a new survey that you see in the list among the other surveys. When you double-click on the survey name you can edit this survey in the same way as for a completely new survey. SeeBasics about designing surveys.
One reason for wanting to make a copy is if you are going to have a survey active for a long time. It could then be advisable to create several separate surveys that are alike, e.g. one per month, as large amounts of data can be experienced as more difficult to handle.