Hello,
Thank you for your post . You can clean up disk by login SSH . if you are not familiar with SSH command  then i can give you another solution .
You can do it by ” ConfigServer Explorer ”  ( if you do not have the plugins you can install it very easily.
http://hosthowto.com/how-to-install-configserver-explorer-on-cpanel-server/
Login your WHM as root user , search the plugins ” ConfigServer Explorer”
Click on ” backup ” folder  .. (  Never click another folder )
Now you can get all of backup files , you can remove unnecessary backups. Do not remove any system file.
NOTE :  : : Please read.  While this utility can be very useful it is also very dangerous indeed. You can easily render your server inoperable and unrecoverable by performing ill advised actions. No warranty or guarantee is provided with the product that protects against system damage.