How to adjust the button "add to cart" on mobile phone

Hello,

Welcome to the shopify Community.

Please remove old given code.

Please Go to Online Store → Themes → Edit code → Assets → theme.scss.liquid and paste this code at the end of this file.

#shopify-section-collection .page-width .grid .grid__item {
position: relative;
min-height: 450px;
}

#shopify-section-collection .page-width .grid .grid__item form {
position: absolute;
bottom: 0px;
margin-bottom: 15px;
}

@media only screen and (max-width: 768px) {
#shopify-section-collection .page-width .grid .grid__item {
min-height: 350px;
}