a[href*="/checkout/"] {
    /* font-size: 0;        hides text visually */
    color: transparent;  /* makes text invisible */
    /* position: relative;  for pseudo element positioning */
}

a[href*="/checkout/"]::after {
    content: "Submit Order";     /* new text */
    /* font-size: 1rem;     set your desired size */
    color: #fff;         /* set your desired color */
    position: absolute;  /* position on top */
    left: 0; top: 0;
    width: 100%;
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: center;
}