Suppose you have some variants like this
You want to change all to Circle Format. You can use this code to Website > Website Tools > Custom CSS.
label[for*="variant-option-"] {
width: 100px;
height: 100px;
line-height: 100px;
display: flex;
align-items: center;
justify-content: center;
text-align: center;
border-radius: 100% !important;
padding: unset !important;
}