Open your presentation and select File. Then Page Setup as shown below.
In the dialogue box that opens select the drop menu next to Slides sized for: and select On-screen Show (16:9) then click OK.
If you have changed an existing presentation you will need to check that any images have not been stretched and resize them.
To change the aspect ratio of your slides in Keynote first [if it’s not already open] open up the Inspector window as shown in the image below.
Then making sure you are on the document tab select your screen size. 1280 x 720 will give you a 16:9 aspect slide.