I try to replace an existing Linux installment with another one, Zorin Lite, on a dual boot PC. After selecting the Linux partition /dev/sda3 (ext3) for installment, I get a message "not root file system" (see the attached image). I did an online search and learned that I need to remove the partition and create a new one. On the creating a new partition panel, I see something like "start space" and "end space" and the selectable devices are quite different than the existing device name /dev/sda1, /dev/sda2, ... . I am wondering whether I am in trouble or not although the Windows still work from the dual boot.

How to go from there?

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that message means you need to specify the partition where the root filesystem will be installed. (it is not enough to have that partition, but you need to tell the installer to use it).

You don't need to remove/delete a partition and recreate, you simply format it. With your Zorin installer, selecting the manual method (Something Else), all you would need to do at the point you show in your image is to select by highlighting the specific partition and then click the Change tab at the lower left of the window and set the filesystem type and the Mount point. The mount point as pointed out above, is what you are missing. Look at the site at the link below which has a detailed tutorial with images and shows exactly this about half way down the page.
https://www.linuxtechi.com/zorin-os-...e-screenshots/

What exactly do you mean by selectable devices are quite different? Specifics please.

Your image has a reference to windows xp, have you checked the Zorin site to see if the hardware on this machine is satisfactory? Maybe the lite version will work?

I have done the same task before. To my memory, I only need to select which partition to install. I thought I already specify the partition, the blue line, for the installment which is the root filesystem, isn't it right?

Yes, in general you are right. Most probably you missed a step somewhere. (I guess you need to click a button or something like that was somehow overlooked).

No, it isnt't right and yes you did miss a step. Explained briefly in my earlier post and in extreme detail with pictures in the link I posted.

I have a look at the article. But, I am not sure that I shall follow the instruction. Again, I want to overwrite the exiting Linux installment. The dual boot has a total of four partitions, two for Windows and two for Linux. I have a dual boot setup a few times and I never had so many partitions just for Linux. I don't know whether that is something special for Zorin OS or not.