If you've done some leatherwork or upholstery - or think you can - then you could do it yourself, but as the others have said - you'll need to refit, restretch & reattach the entire panel, not just the top bolster that's come undone.
If you're new to it, then get some glovers & upholstery needles, upholstery thread/cord & some scrap vinyl of similar weight & practice to see if you can do it yourself. Otherwise take it to a good upholstery shop locally & give them pix &/or a solid sample 914 door panel to compare to so they do it right.
Word of caution - from your sideview mirror & aluminum threshold - I'm guessing that yours is a 70-71 MY, & the leatherette fabric varies from some MYs - so as long as you're going to do them, you may want to see that they're both done in the proper smooth, perforated &/or basketweave vinyl leatherette for your MY. See this for samples of most of them:
http://bowlsby.net/914/Classic/PaintFabrics.htmPS - Great How to post Johannes - that will help both corsepervita & I!!!!