You can delete them using a PC if the Edge is mounted. The activities are located in the Garmin\activity folder on the Edge.
It’s a shame that I can’t delete all rides at once in my Garmin 130 plus .
This topic is about deleting previous rides (activities) stored on the device. Too many stored eventually slows down the Edge 130 response. Deleting on the unit itself is slow and painful, but it's not a big deal just to plug into a computer and clean it up as per aweatherall.
This is just regular maintenance of your Edge, plug into a computer and empty out the past activities you don't need on the device. Just like any computer file folder, you can select as many or as few as you desire to delete. Although we have some things to complain about as Garmin owners, this isn't a big one. I see connecting to my laptop as just a normal part of ownership. That said, the owner's manual could absolutely be better about helping with things like this. Owner's manual is terrible and this forum is the only thing that has helped me fully understand things like this.