This article is for cPanel Accounts. Determine where your account is with this guide.
You cannot modify the server's root php.ini file. However you can make your own php.ini file that will override certain php variables.
You just need to log into cPanel and go into your File Manager. Then navigate your way to the directory you want the php settings changed for and click on the 'new file' icon. Then name it 'php.ini' (without the quotations). Then you need to edit the file and you can place in the variables that you want.
//Common local changes:
upload_max_filesize = 20M; // (default 8 - Max, 32)
post_max_size = 20M; // (Average, 20 - Max, 32)
register_globals = Off; // (off by default - you can turn On)
allow_url_fopen = On; // (off by default - you can turn to On)
memory_limit = 24M; // (default of 8M, Max 32)