Opening a strong closed guard in the Gi is usually very tough.
You can either stand explosively and use both hands to push open the legs or you can pass from the knees.
For No Gi you can try this one:
Branko starts off by opening the opponent’s lapel and passing it under his back. He places a closed fist right in the center of the spine, forcing a huge amount of pressure.
The pressure will make the opponent very uncomfortable and will distract them from keeping a closed guard. Once Branko opens the guard he staples the legs and passes easily to side control. It’s important that you release the lapel control once you pass the guard as you can get your arm stuck underneath.