Looks like you want:
$update = "UPDATE paceReport SET `$fieldname` = '$content'";
However, if the variables are coming from any sort of user input, then you should "sanitize" them to prevent SQL injection attacks and such. One method would be:
$update = sprintf(
"Update paceReport SET `%s` = '%s'",
mysql_real_escape_string($fieldname),
mysql_real_escape_string($content)
);