I can't take SIM card out of my iPhone 6s. Please help

You are using the sim removal pin which came with your phone and are pushing it into the tiny hole with some force?

Are you asking what to do before selling or trading in your iPhone or what to do to send your phone in for service?

If you are trading in the phone --> What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

What to do before sending in for service --> Get your iPhone, iPad, or iPod touch ready for service - Apple Support