Which model do you have?
The watches I've seen have the strap moulded as part of the body.
If yours has the straps such that they are removable, then look where the strap meets the body and using a rigid thin blade (pref blunt so you don't cut yourself), insert this at right angles to the strap and turn slightly such that the retaining spring-loaded bar (which holds the strap in palce) pops out.
Be careful - as the bar is spring-loaded it can fly off in any direction.
Usually to remove the back off the watch one looks for a slight depression on the watch body and then slip something flat, narrow and rigid under the back plate and flip up.
Another warning - there may be a seal between the back plate and body to make the watch waterproof, be careful when taking the back off so this does not get lost.
Assuming all goes well, when re-assembling the parts the back plate should go in precisely the same position.
Good luckHow to change the battery in a nike watch? (bowerman series)?
Talk about a piece of junk. Why would a company like Nike make a Bowerman watch which requires surgery to replace the band? I have never seen a watch (and I am old) that didn't have a quick way to replace the band.Shame on Nike.