I hate to say this, but unfortunately there really isn't a good way to stretch sewn-in elastic except time and heat (in the dryer). If you're talking about elastic which is encased in a seam around the waist, you can open up the seam and try replacing the elastic inside with a longer piece of elastic. My tights have the encased elastic, so they would be easy to do, but I don't have any experience with Under Armour...
East Hill
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TRY EMPATHY & HAVE LOVE IN YOUR HEART, PERHAPS I'LL SEE YOU ON THE ROAD...