Most webhosting providers will allow you to change (some) php settings via .htaccess files. Here is what you need to add:
php_value post_max_size 52428800 # 50MB
php_value upload_max_filesize 52428800 # 50MB
php_value max_execution_time 300 # 5 minutes
After making these changes to .htaccess file, double check using:
<?php phpinfo(); ?>
Look at the local values.
PS: This works if PHP runs as an APache module. If your web hosting setup is different, you must ask them about how to do this. AFAIK, it is possible to put (some of) the settings in a php.ini file and upload to the root directory of your website but depends on how PHP and Apache are setup.