Removing the previous button and center align the submit button at last step form

ff-last-step-form-buttons-alignment.css

/* .fluent_form_18 is my form class. youe will replace form ID 18 with your form ID */

.fluent_form_18 .fluentform-step:last-child .ff_step_nav_last {
    display: none;
}

.fluent_form_18 .fluentform-step:last-child .ff-t-column-2 {
    display: table-row;
}

.fluentform .fluent_form_18.ff-form-has-steps .ff_submit_btn_wrapper {
    text-align: center;
}