One Product
#1. Hover on the product that you want to rename, click on the (…) icon and choose Edit

#2. On the left of the screen, you choose Checkout and then choose Custom Button

#3. Next, you enable Custom Label and set a new name at Label

Result:

All Products
You add this code to the Custom CSS box
/* rename add to cart - all products */
.sqs-add-to-cart-button-inner {
font-size: 0;
}
.sqs-add-to-cart-button-inner:before {
content: "new text";
font-size: 20px;
}

Result:
