You can read the commented lines to understand the code, it’s quite simple!
Paste this in your taxonomy.php!

 <?php
    //get the global term SLUG from $wp_query variable..
    $term_slug = get_query_var('term'); 

    // get all info related to that term
    $term_info = get_term_by( 'slug',$term_slug ,'YOUR_TAXONOMY'); 

    // this line converts to integer the id of the parent cat!
    $parent = (int)$term_info->parent; 

    //Use get_term_link() function to print the correct link to the parent taxonomy, 
    //we also CHECK if there is any parent, otherwise, do nothing..
    echo ($parent)?'<a href="'.get_term_link( $parent , 'YOUR_TAXONOMY').'">Go Back</a>':''; 

?>