Form Block – Rename First/Last Name

Use this code to Custom CSS box

div.first-name .caption-text {
    visibility: hidden;
}
div.first-name .caption-text:before {
    visibility: visible;
    content: "First First";
}
div.last-name .caption-text {
    visibility: hidden;
}
div.last-name .caption-text:before {
    visibility: visible;
    content: "Last Initial";
}

Result: