reinstall win 10 dual boot system

I need to reinstall win 10 without deleting Ubuntu OS
I do not know how to tell windows to overwrite only old win 10 partition and not entire hd
Thank You

12 Click/tap on the Custom: Install Windows only (advanced) option

Drive options (advanced)
If you have more than one partition for a Drive # (hard drive) listed and want to install Windows on only one of the partitions and keep the other partitions with that disk #, then only select the partition that you want to install Windows on the Format option.

How to Clean Install Windows 10
Clean Install Windows 10

Once Windows 10 is installed the computer will only boot to Windows 10. You can use EasyBCD to fix this. Download the "Non-commercial Free" version of EasyBCD. Run EasyBCD and add Linux to the boot menu.

EasyBCD
EasyBCD - NeoSmart Technologies

Boot from the Windows 10 USB flash drive. Go through install options and select the Custom Install Option. On the next screen you will see a list of drives of and partitions. Click on the NTFS partition that you want to install Windows 10 to and click next.

Also this tutorial may be useful:
Dual Boot Windows 10 with Windows 7 or Windows 8

I see 2 windows partitions: system and primary
I select primary partition during installation
Does Windows 10 know to use both of them?

robzane said:

I see 2 windows partitions: system and primary
I select primary partition during installation
Does Windows 10 know to use both of them?

Windows setup should install Windows to the primary partition and update the system partition with new boot files.