Backup everything near and dear to you.
Install Ubuntu on a flash drive. If you use the ubuntu startup disk creator be sure and leave some room for adding software in the reserved space (near bottom of GUI).
Start computer from the Live Ubuntu and connect to the internet.
Install Gparted onto the flash drive. sudo apt-get update then sudo apt-get install gparted
Launch Gparted and reduce one volume as per your preferences. Most likely it will be in front of the Ubuntu partition.
When that finishes you can either
A:Create a new partition that you use as a data partition which makes backups and new installs a lot easier in the future, is fast to do and safer.
or
B: Move the Ubuntu partition over and then expand it. DO NOT STOP THE PROCESS ONCE STARTED!!! You will regret it if you do. Show patience, it takes time.