Can't get iPhone 6s open

you need a pretty thin metal spudger to pry under it, anything plastic or too flimsy you’ll have problem with if the glue is still strong. If you can get under it at the bottom left a small bit then try pushing around the side with it just make sure you don't put the spudger too far in to avoid catching internals. or wedge a second spudger or something thin like a guitar pick or something in any gap you can make an then try pry in another spot near it. if you are going to use head then you don't need too much, just enough to soften the glue. If the glue still isn’t for budging I would try a thin Stanley knife blade (using just the tip) to try cut through the glue almost.