if you want to cancel and stop with A-kasse you must contact your A-kasse and tell them that you want to stop.
You must be aware that if you stop paying for A-kasse, you will loose some of your earned rights - this applies to the membership period. You must have been member of an A-kasse for at least 1 year again if you want to receive unemployment benefits.
If you want to switch to another A-kasse, you must sign up with the new A-kasse, before you cancel with your current A-kasse. The new A-kasse will take care of all the practical matters, this includes that you will pay membership fee in only one A-kasse for the same period. The two A-kasser will also take care of having your earned rights (working periods/income and membership periods) registered correct in the new A-kasse.