Offline files are stored in the \windows\CSC folder
To delete the offline files in CSC Folder, you will first have to disable Offline Files. Then, you can change the permissions of the CSC Folder and its subfolders and delete them.
Step 1: Disable the Offline Files.
a. Open Sync Center and click on Manage Offline Files on the left.
b. Select the Disable Offline Files Button and reboot the computer.
Step 2: Give yourself all the permissions.
a. Press Windows key + X and select Command Prompt (Admin).
b. Type these commands and press Enter after each one.
cd c:\Windows
takeown /f csc /r /a /d y > NUL
icacls csc /grant Administrators:(F) /t /l /q
c. Delete the folders under C:\Windows\CSC.
Note: You will need administrator access or password to perform these steps.