Folder Options window
Now to change the setting to Single-Click you will just need to click on the Single-Click to open an item button. After you have done that just click on Apply and now you don't have to double-click on files to open them anymore. Also since you open the files with one click now you can't select them with one click. So what you need to do is just scroll over the files to select them. To change this setting back you just need to go back to Folder Options and select the Double-Click to open button.
Single-Click To Open selected