First of all, you need a CD/DVD or a removable USB device which contains the installation files of the operating system. If you don't have any of those you will have to download the installation files from the Internet and burn them to a CD/DVD or make a bootable removable USB device.
If you don't know how to burn a CD/DVD or make a bootable USB device, check out the don't know how to make a bootable (burn) CD/USB solutions.
Once you have a CD/DVD or a bootable USB follow the steps below:
Choose a partition, click Format (for every partition separately) and all data stored in it will be deleted -> Click OK to confirm -> select a partition again -> Click Next
Note: Don't interrupt the process and be near the computer in case if something unexpected happens. Don't be alarmed if the computer restarts because that is a part of the process.
If you use a laptop, connect it to the AC adapter to prevent it from shutting down.
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