How to modify template tags function?

There is no get_user_meta filter. Despite WP often having filters same as function name, it isn’t the rule exactly.

There is however deeper filter in more generic get_metadata() function:

$check = apply_filters( "get_{$meta_type}_metadata", null, $object_id, $meta_key, $single );

if ( null !== $check ) {
    if ( $single && is_array( $check ) )
        return $check[0];
    else
        return $check;
}

Also your PHP is a mess with callback there. All together:

class modifyUserMeta {

    function modifyUserMeta(){

        add_filter('get_user_metadata', array($this, modifyMeta), 10, 4 );
    }

    function modifyMeta( $null, $object_id, $meta_key, $single ){

        // check ID, key, and format then return accordingly
    }
}