When we scroll down and WC sticky navigation comes it shows “select options” which goes on two lines on mobile devices. Now, I have two options, either I change the size of the text which will be implemented on all buttons, and the second is to change the text.
GP returns the add_to_cart_text variable that is set by Woocommerce.
You can change its string using the woocommerce_product_add_to_cart_text filter hook.
See my reply below for an example for changing that string on a variable product: