Assume it will damage files in the Windows partition even though it likely won’t (things can go wrong, but more likely - we all make mistakes sometimes and a simple incorrect click or confirmation could wipe everything). So back all your data up and then do it. Probably worthwhile downloading Windows installation media too in case you have problems.
If you put your Ubuntu bootable usb in and restart with boot priority set as usb first (in the system’s bios / uefi settings), you should get to the install options.
The Ubuntu installer will usually give you the option to install alongside Windows which is what you want. If it doesn’t, you can select ‘something else’ and install it in the partition that previously held your 18.04 installation. You might want to consider if it will be easiest to start from the beginning and deliberately wipe the drive for a fresh windows install (leaving unallocated space for Ubuntu), and fresh Ubuntu install with all your partitions planned in advance. I guess that depends on whether you’re happy with the size of the previous Ubuntu partition to hold the new installation as well as your comfort level with that kind of thing.
