How do I change the goggles' strap?
To start, unbuckle the strap from the plastic clip in the middle. Unthread one side first and thread the new strap through that side. This way, you can use the threaded side as the reference for the new strap.
Once the strap is threaded, always fix the end of the strap back down to the clip to prevent slipping during use.
You can purchase a replacement head strap here: How do I order replacement parts?