There are four T4 screws located on the back of the watch that are removed to allow the two halves to come apart (Circled in Picture). The screws have a little of the silicone gasket around them that you may want to clean of to make re-installation easier later.

Though it appears to be designed for a T4, I found it easier to work with the T3.

Once the screws are removed you will need to separate the top and bottom. There is a silicone gasket/seal that is holding the two halves together. I was able to use the blade of a hobby knife in a small gap at the top of the watch, closest to the 4 holes (Arrow in Picture).

This appears to be the safest starting point since the sensor you are most likely to clip, if you slip through the gasket quicker than expected, is protected by a metal casing.

As a gap is created insert another flat pry tool, into the gap so you can use the two to work your way around the edge gently cutting or peeling away the gasket/seal.

There are four plastic clips that help hold the top and bottom together without the screws.