Really just pull and pull hard. But if that's giving you trouble you can use a flathead screwdriver to wedge it in and gently pop it out. Be careful though because the clips can scratch up the rims if you take it on and off a lot.
Just reach inside a gap in the plastic cover toward the outer ring, get your fingers behind the cover and give it a good tug toward you. It is just held on by a wire ring that fits inside the alloy wheel. Be careful not to lose your balance when it comes off !
Thank you! Do I pull from one corner or from two opposite ends (180 degree apart)? (I don't have strong arms.)
Insert one side, then hit the other side hard with the palm of your hand. Just pull from one side. If you are not able to pull the cover off using a hand then use a large flat-blade screwdriver to exert prying force.
When I took my Touring rim covers on and off, I first used a rubber mallet and a towel to gently hammer it in. Over time, though, I just used my hands to push it back in there. The more you do it, the easier it becomes without any special tools. Same goes for pulling it off.