Change newsletter button to lowercase or capitalcase

Example:

Code:

/* -- code to newsletter button to lowercase -- */
.newsletter-form input.btn {
    text-transform: lowercase !important;
    padding-left: 1rem; padding-right:1rem;
}
/* - end - */

First letter capitalize version:

Code:

/* -- code to newsletter button to capitalcase -- */
.newsletter-form input.btn {
    text-transform: capitalize !important;
    padding-left: 1rem; padding-right:1rem;
}
/* - end - */

Where do I add the code?

Use this link to learn where exactly to add this code snippet:

Where to add your CSS style code