Yes - as long as you're an event admin.

Simply find their application in the applications section of Eventree.

Then click the down arrow to the left of it and select 'Edit application':

This will open their application form exactly as they submitted it. You can make any changes to the form as if you were the applicant themselves. When you've finished simply click 'Save changes' at the bottom of the form.

Note - the applicant will receive an email telling them that you've edited their application: