To download form results you must have the form set to save results to an excel file. If the form has been submitted a results file will have been created within the folder that was selected during the setup of the form. You can double check where the file is saved by clicking the edit button on your form page and looking in the "If saving results to cms" section under "Choose a file in the folder in which you want to save the file".
Once you have the location of your file you can navigate to it via the list of folders on the left. Once you have located the file click on it to open up the preview pane. From the preview pane click the download link.
You can download any content item that is of the type "file" in this same way. This is anything with the orange file icon. This includes images.