Of course you can. If you re-cut your trailer or want to try out different trailers to see which one works best, you can change your trailer at any time.
Just go to your film page on Distrify and click "Upload Video" in the right-hand column. Select the trailer video file from your computer and upload it.
Once it has finished encoding (it shouldn't take more than a few minutes), go back to your film page and click "Change your player settings" above your player. On the next screen select the new video from the Trailer menu at the top of the page. Then scroll down to the bottom and click "Save Changes".
As with all changes you make to a Distrify player it may take a few minutes for it to appear in the player window, and you may need to reset your browser cache to see the changes. For more on this see http://support.distrify.com/customer/portal/articles/266945-my-changes-aren-t-showing-up-in-my-player-what-s-happening-