To change the PHP maximum upload size, you need to edit the php.ini file and then restart the webserver.
Depending on your Linux distribution, the php.ini will be located at a different location:
CentOS / Red Hat: /etc/php.ini
Ubuntu / Debian: /etc/php/your_version/apache2/php.ini
You need to change two parameters: upload_max_filesize and post_max_size. The following setup will allow you to upload files up to 20MB:
upload_max_filesize = 20M post_max_size = 20M
After saving the file, restart the web server:
sudo service apache2 restart