Use media queries. http://www.w3schools.com/css/css_rwd_mediaqueries.asp
You have to make the button different on desktop, tablet and mobile.
To do that, you will ned to set media queries in your css file.
You can find the queries here: https://www.w3schools.com/css/css3_mediaqueries_ex.asp
If you don't have any experience with coding, I will recommend you to find a developer. Should be 2-5 minutes work.