Uploading, downloading, editing?
I assume uploading.
Are you on wifi? Sometimes if the signal drops out it struggles to pick back up where it left off.
Two main options
1) Stop it and try again, preferably with a better connection (wired into the router works well but if it is just move closer to it then so it goes). You might get lucky and it picks up from where it left off.
2) Stop it there and if it is only 86% then make a new version of the video with the remaining 14% and stitch the two together. However I am told they don't have that much editing ability these days compared to what they had.