Override class inside $atts shortcode_atts in WordPress

I have a plugin and I need to tweak one function. Specifically the ‘class’ => ”.

This is the code in the plugin:

function message_to( $  atts, $  content = null, $  tag = '' ) {     $  atts = shortcode_atts( array(         'to'        => '{current-post-author}',         'subject'   => '{current-post-title}',         'text'      => __( 'Contact','front-end-pm' ),         'class'     => 'fep-button',         'fep_mr_to' => false, // Comma separated list of user ids (used in PRO version)     ), $  atts, $  tag );     if ( '{current-post-author}' == $  atts['to'] ) {         $  atts['to'] = get_the_author_meta( 'user_nicename' );     } elseif ( '{current-author}' == $  atts['to'] ) {         if ( $  nicename = fep_get_userdata( get_query_var( 'author_name' ), 'user_nicename' ) ) {             $  atts['to'] = $  nicename;         } elseif ( $  nicename = fep_get_userdata( get_query_var( 'author' ), 'user_nicename', 'id' ) ) {             $  atts['to'] = $  nicename;         }         unset( $  nicename );     } elseif ( '{um-current-author}' == $  atts['to'] && function_exists( 'um_profile_id' ) ) {         $  atts['to'] = fep_get_userdata( um_profile_id(), 'user_nicename', 'id' );     } else {         $  atts['to'] = esc_html( $  atts['to'] );     }     if ( false !== strpos( $  atts['subject'], '{current-post-title}' ) ) {         $  atts['subject'] = rawurlencode( str_replace( '{current-post-title}', get_the_title(), $  atts['subject'] ) );     } elseif ( ! empty( $  atts['subject'] ) ) {         $  atts['subject'] = rawurlencode( $  atts['subject'] );     } else {         $  atts['subject'] = false;     }     if ( empty( $  atts['to'] ) && empty( $  atts['fep_mr_to'] ) ) {         return '';     }     return '<a href="' . fep_query_url( 'newmessage', array( 'fep_to' => $  atts['to'], 'message_title' => $  atts['subject'], 'fep_mr_to' => $  atts['fep_mr_to'] ) ) . '" class="' . esc_attr( $  atts['class'] ) . '">' . esc_html( $  atts['text'] ) . '</a>'; } 

The original function specifies the class:

'class'     => 'fep-button', 

I need to change it to something else:

'class'     => 'something-else',