PHP upgrade

How do I upgrade my site,,
to PHP 5.5? It is currently running on PHP 5.3 and It has a bunch of goobly gook above the header that I would like to get rid of -
Strict Standards: Declaration of Tag::fieldIsUnique(), etc.

The problem isn't really with the version of PHP.

Upgrading Omeka is probably the better option, since it looks like it is a fairly outdated version of Omeka. The current version should make those messages go away.

Updating the PHP version _might_ also hide those messages, though the underlying issue would still be there. A PHP upgrade is something for your server admin to handle.

More fundamentally, it's a matter of the settings for PHP error messages. Also a job for the server admin, but removing the E_STRICT error logging level would also hide those messages