My battery wend dead. the store will not open the back of the watch to change it. how do i open the back to change the battery?

Thank you for contacting FixYa.
Most Skagen watches have a snap-in, snap-out back. You will need a case back opening tool ($3 to $14) or a slotted jeweler's screw driver or knife blade to pry it open. Do this over a counter or towel in case the battery pops out. Examine and make note of which side of the battery is up. Get the number off the battery, better yet take it with you to Wal-Mart, Radio Shack or anywere else they sell batteries. Buy a replacement, install it, and try to reinstall the back. Good luck!
Do not try placing your Skagen watch face down on a soft towel, on a hard surface, and then then try to mussle in the back, or tap it with a mall or hammer. You very likely will break the crystal.
Without a Case Closing Plier ($15 to $20) or a Case Closing Press ($15 to $50) you will not be able to get the back to snap back in. You could try a conventional vise and size some vinyl or plastic washer to to fit the bezel and protect the crystal/face of the watch, and some flat vinyl/plastic that covers the back of the watch. Align the front and back plastic pieces with the back, carefully insert it in your vice, and tighten until the back snaps in. This could be fatal if you mess up!
A simpler solution is to take your watch to a place that specializes in selling and servicing a variety of watches, hopefull Skagens, maybe a jewelry store, and have them replace the battery. They should have the closing tool and you can expect to pay $10 to $15 for the battery and installation.
If you like Skagens, you can go on-line and find the appropriate tools to do this. Or, pay the watch store. Many Skagens use up batteries at the rate of every 10 to 15 months. Also, you can look up Skagen on line and see if they will service your watch. They are located in Reno, Nevada