How to release Planet Bike fender safety release tabs
The only purpose of the tabs is to be the weakest link in the chain so that, if something gets stuck between the wheel and the fender, the tabs will break, allowing the fender to come loose. If you didn't have the tabs, the debris would wedge between the fender and wheel, locking the wheel and throwing you over the handlebars.

You never need to release the tabs deliberately. To detach the fenders, pretend the tabs don't exist and just unscrew the bolt that holds them near the axle.