This is the only replacement that I could find.
https://rounded.com/samsung-gear-sport-sm-r600-back-cover-gh82-15357a.html
If you do not have one, you will need a [https://www.amazon.com/Tri-point-Triwing-Magnetic-Screwdriver-Disassembling/dp/B01MRPFSS5/ref=pd_lpo_sbs_469_t_0?_encoding=UTF8&psc=1&refRID=NT9211BQ2ZMXTXPYFP5T|tri-wing screw driver]. You will also need a [https://www.amazon.com/s?k=pry+tool&i=tools&ref=nb_sb_noss_2|splunger] or a guitar pick or playing card to pry the back open once you remove the screws.
[https://gethypoxic.com/blogs/technical/samsung-gear-sport-teardown|Here] is a simple tear down guide for the Samsung Gear Sport.
Hope this helps. Good luck.