Securely hold the watch head
To remove the strap – gently compress the strap creating a gap between the lug and strap. Use the strap spring bar tool (provided when you purchase your watch) and insert it having the "V" shape area on the groove of the spring bar. Gently push the spring bar pin in the direction of the strap to release it from the lug and you'll be able to pull the strap off.
To attach the strap – insert the spring bar pin into the strap with both ends of the pin protruding out. Place one end of the pin into the hole on the lug. Using the spring bar tool, insert it into the groove of the pin and compress it inwards. Thereafter, adjust the strap in position before releasing the pin. The strap should now be in place and securely attached.