2513 wv 100

This is a simple spring clip. At the end nearest the wrist strap there is a hook clip. Insert a small, thin pin and hook the lowest edge outwards and the strap should spring free.
Try the video link below, (at 2:45 minutes). It's a slightly different watch but the method is the same.

https://www.youtube.com/watch?v=8GI-xfBXHj4&t=261s

Go to Casio's web site to find a service center near you. I think Atlanta is the closest, but they may have opened one in/near Miami.

here are some videos that may help you

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwi3ubnCjK76AhUYBDQIHdsDAbwQwqsBegQIBRAB&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DeRy0JlsjfqQ&usg=AOvVaw3hOvdMy7BXU6XfkxuFHigB

browse through my tips and you will find link for all Casio watch manual downloads.

after calibrate sensor display numeral are blank

Check out my TIPS here and you will find out how to do that

Yes, the flashing battery icon tells you it's time to change the battery.
Despite of that the watches are fitted with brand new batteries when they are leaving manufacturer, there can be a significant period of time till they reach their new owners and the watch battery can be flat (or allmost flat) at the selling point.
I suggest you visit sellers shop again a.s.a.p. and ask for the brand new battery to be fitted under the warranty.

Rate me, please.

I just replaced the battery in my 20yr old Seiko chrono sports 100. And I had the same problem. I think I stopped it on 12 while in stopwatch mode and held down the start stop button and the set button at the same time. See if that works dude.

you can download the manual on this website:

http://www.casio-radio-controlled.com/euro/support/

for the WV-100 the code is 2512