In order to change the file size limit for PHP, follow the below steps:

Log into Control Panel with the details given to you in your welcome e-mail.
Go to "Select PHP Version". It may be easier to search for "PHP" in the search bar.
Select any non-default version, and click "Set as current".
On the top right, click "Switch to PHP Options". 
Click "upload_max_filesize", and select your preferred value.
Finally, click save.