1. Log into your Plesk control panel
2. Go to the “Websites & Domains” menu item on the left hand side
3. Click through the the “PHP Settings” option
4. Scroll down to the “Performance and Security Settings” and change the post_max_size field and the upload_max_filesize field to be the new max size you wish.
5. Scroll to the bottom of the page and press “OK”