Go to settings of your coming soon template. At the top of the page there is blue panel with coming soon settings. You need to find “More” button which will open the additioanal settings. Click on it. There you will be able to change the value of the countdown timer, find “Date Start” (it is the day when you decided to enable Coming Soon page) and “Date End” (day when you will finish your site development and disable Coming Soon mode) – http://prntscr.com/9yw39y
Simply change these values on desired. Please try it!