Drupal 8 Webform - Change to Dev Version
if you have drush on your server you can easily select a version with this command

drush dl webform --select


it should show something like this

Choose one of the available releases for webform:
 [0]  :  Cancel                                                    
 [1]  :  8.x-5.x-dev     -  2017-Sep-07  -  Development            
 [2]  :  8.x-5.0-beta16  -  2017-Aug-08  -  Supported, Recommended 
 [3]  :  8.x-4.x-dev     -  2016-Nov-28  -  Development


and you could press 1 then enter to select 5.x-dev

i believe you'd also be prompted with a message warning you that you're overwriting the previously installed beta16 version and you'll say ok