Can I DIY-repair an adjustable elastic waistband that has detached?

I bought my Patagonia coat only a couple of months ago and a few weeks ago I noticed the elastic waistband was simply hanging from the side of my coat. It seemed to have detached from the interior of the waist section of the coat. I was thinking of sending it in to get repaired but noticed that you help customers repair stuff themselves. I would be happy to do that if someone explained how that could be done without me ruining my coat.

Thank you in advance,

Max