That error basically means that there is some output to the page before the session start function is used.
according toi the error:
output started at /var/www/html/new/Admin/Products_Update.php:6
the ouptput is arund line 5 or 6.
Usually it is either a blank line between ending php code block and a starting php code block:
or it can be a space after the end of the php code block:
For more details, see the following