You can modify PHP's upload_max_filesize as per your needs using CloudLinux's PHP Selector in Plesk.

1. Log in to your Plesk account.

2. In the right-hand sidebar, click on PHP Selector.

3. Click on the Options menu.

Note: If the Options page shows an error related to the native PHP version, change it to a non-native version first.

4. Look for upload_max_filesize and click on the drop-down option displayed in front of it. Then choose the appropriate limit as per your requirements.