To modify the storage allocation method for SharePoint sites, follow these steps:

1. Navigate to the SharePoint Admin Center (https://admin.microsoft.com | Admin centers | SharePoint) and select Settings from the navigation menu on the left.
2. Select Site storage limits on the Settings page.
3. Select Automatic to allow SharePoint online to manage the storage automatically, or select Manual to configure the limits for each site:

4. Click Save when you're finished.

Switching from Automatic to Manual for the first time will set the site's storage quota to the maximum storage size of 25 TB (regardless of how much storage you have available in your tenant). This is still essentially the same as automatic. Then, you can adjust the sites so that they meet your business requirements.