It is controlled by /etc/php5/apache2/php.ini. You need to find it and modify these lines:

memory_limit = 128M (no change needed here, it was already set at 128M)
post_max_size = 60M (it was set at 8M)
upload_max_filesize = 60M (it was set at 2M)

Remember to restart apache2 and refresh your wordpress page after doing it.