I have my laptop setup as windows 10 and plasma dual boot and I want to switch it to just be a single boot plasma setup. I know partitions are really important and I could mess up a lot of things, so do I just need to delete the windows partitions and resize the plasma one in the partition manager or do I need to do things in the terminal as well.
You can delete the windows partition from GUI and extend the linux partition from GUI (Would suggest you to install GParted - I found it to be faster than KDE Partition manager).
You need to change your Grub to remove Windows from the list of OSes. Else it will keep on showing (but of course, wont boot).
Okay thank you for the help.