There is no guarantee that you won't "break" Windows, especially if you are using legacy BIOS and especially if you aren't familiar with installing operating systems all the time. Make sure that you back up all your important data. Even the best of us make mistakes sometimes. It's probably a good idea to also make a Windows installation USB. Worst case scenario is that you mess up and you have to reinstall- and you can finally switch to UEFI/GPT which will remove the restrictive 4 partition limit which is really not conducive to dual booting anyway.
There are a few ways to go about this, but if I were you I would use the installation media to boot a live session (Try Ubuntu). Use the Disks application to delete the partition where Ubuntu is currently installed so there is unpartitioned free space.
Then start the installation and you should get the option to "Install alongside Windows" as long as you have booted the USB the same way Windows is installed (in Legacy BIOS mode). You will need to consult with your motherboard documentation if you are unsure how to boot the USB in legacy mode if you aren't sure-- all motherboards are different.
