Reseller Package Modification Not Working

I own a reseller hosting with 100GB space. There's still 9.35 GB space left. One of the package is assigned 35 GB space and only one site is associated with this package. So, now I want to increase it to 40 GB. I do this :

• I open my WHM account.
• Go to 'Edit A Package'.
• Select the 35 GB package.
• Modify the Disk Quota to '40000'
• Save Changes.

But nothing changes. The package is still assigned 35 GB. Can anybody please help? I looked everywhere but couldn't find anything.