XPS 17 L702X: How do I change AHCI to ATA?

WHY do you want to?!  Not all systems give you the option, as it is a legacy setting that can affect system performance.

I want to BECAUSE I'd like to install Windows beside Linux

You can usually install modern Windows and Linux OS's side by side in AHCI mode.  The manual doesn't seem to mention System Setup options for reference, but it would normally be found under SATA Operation, Storage, or Integrated Devices, again, IF it even has that legacy option.

I don't have any trouble running Windows and Ubuntu side-by-side with an AHCI boot device.

The problem is that I have installed already Linux and want to install Windows now.

There should be no problem installing Windows at this point.  What version of Windows are you wanting to install?  If you are experiencing a problem, what it is it, exactly?  If you have yet to experience an issue, then just boot to the disk to install (keep in mind this will break the GRUB bootloader).

How did you format the disk when you installed Linux?  Windows 7 MUST be installed to an NTFS partition, so you need to DELETE the partition you want to install Windows to so that it shows up as Unallocated Space in Windows Setup.  Also, is the disk GPT or MBR?  Are you wanting to install 7x64 or 7x86?  When it says it can't install to this disk, can you click Install anyway?

I made a 2nd partition formatted NTFS. I want to install 7x64 and I can't click install anyway