Hey ! 
To my knowledge, the only explicit mention of how one of the 12 disciples was martyred is about James, in Acts 12:2, where it says that he was put to death with the sword.
For the other apostles, we only know about how they were martyred through the writings of other people who witnessed it with their own eyes. 
Although these writings are not part of the Bible, they still represent trustworthy ressources, that can be refered to with the appropriate caution and critical mind. 

@shinjitsunokenth Thank you I will first look at the book of Acts then

Stephen's martyrdom can be found in Acts 6

@shinjitsunoken Thank you for sharing!!