Fatal Error: Memory Allocation

I received the following error when trying to upgrade to WP 3.8.1:

Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 208640 bytes) in /home/kingxxxx/public_html/wp-includes/functions.php on line 3944

Where do I need to increase the allowed memory size to address this issue?

Regards,

Martin