The Odyssey's trunk hatch is too heavy for most people to open it by hand. So two pressurized struts help to easily raise the hatch, and also to support it in the raised position.
Over time, a strut can lose its pressure. The hatch then becomes more difficult to raise (this was my problem), or it may not stay raised on its own.
A weak strut could create a safety hazard if it suddenly fails when the hatch is in the raised position. A person standing under the suddenly closing hatch could be injured.
Replace both struts if you notice that the hatch is becoming more difficult to open. Both won't fail at the same moment of course, but replace them in pairs anyway because if one has failed, the other is likely to fail soon.
IMPORTANT: Make sure to securely prop open the trunk with something sturdy. (if not have someone hold open the trunk)
Use a small flat head screwdriver to pry open the clip. Insert the screwdriver head into the smal opening. (this picture shows the clip closed)
Once the clip is open, simply pull the strut away from trunk. It might require a little force, but should easily come off. You may have to lift the trunk a little.
Repeat for the bottom strut. Open clip and remove strut.
Adding the new strut is easy. The strut should just snap into place. You should hear a click when its on properly. Make sure to wiggle it a little to make sure its on.
You may have to lift the trunk to get the strut to fit at both ends.
Repeat on other side. I would recommend replacing the struts as a pair.