connected-form-back-button.php

<?php
add_filter( 'caldera_forms_render_get_field_type-cfcf_back_nav', function( $field, $form ){

	//change your form ID to target a specific form
	//remove conditional to target all connected forms
	if( 'cf123456' == $form[ 'ID' ] ){
		$field[ 'label' ] = 'Next Step';
	}

	return $field;
}, 10, 2 );

connected-form-next-button.php

<?php
add_filter( 'caldera_forms_render_get_field_type-cfcf_next_nav', function( $field, $form ){

	//change your form ID to target a specific form
	//remove conditional to target all connected forms
	if( 'cf123456' == $form[ 'ID' ] ){
		$field[ 'label' ] = 'Previous Step';
	}

		return $field;
  
}, 10, 2 );