Hi! I am running w10 and UM 18.04 on a Lenovo laptop and I want to re-install w10!from scratch. Is that possible?
Or would I need to set up the whole system again?
Yes, you can. But then you need to reinstate GRUB as bootloader, or if you use the NT bootloader to add the instance of Ubuntu MATE into that. Since you're doing it from scratch, consider using a smaller partition for /home so you can use a common partition for your personal effects. Both systems can read from it proper if you take the time to set things up post-install.
That's what I do, going strong so far aside from the occasional NTFS hiccup requiring me to chkdsk in Windows. But that's only happened like what, once in my years using the system? Then when one day my UM installation was somehow obliterated I was just able to reinstall it and set it up once more.