Boot your PC from the bootable USB, then select the Custom: Install option.
On the next window oyu will see the partitinos on oyur drive, you need to delete the EFI (Boot) partition, the C drive partition and the Recovery partition, they will merge into one large Unallocated Space, select that space and click 'Next' to begin the instalaltion.
Please Note, this process will delete all data from oyur C drive.