What version of Windows do you have installed? I don't think the software assistant works with Windows 10. I have found a lot of programs are not very compatible with Windows 10, so I uninstalled Windows 10 back to Windows 7 Home Premium and they all work. Hope that helps.
I updating my One M8 to Marshmallow using Win. 10 it went ok it took a while I used HTC's sync Manager it Downloaded the update through Verizon's phone Repair..
It went through Ok although I had to Run a System Cache Clear shortly after the Update as the phone was Running a little Rough after doing the the Cache Clear the phone is operating Fairly Good one Improvement my Battery I was only getting 4 to 5 of it on a Full Charge Now I can get almost what I got when it was New 10 to 20 hours Depending on how I use..
On your Device I'd make sure you have Plenty of Room to install a Big update as this one Clear any unwanted Files Pictures, Videos, Music anything that might consume a large amount of Internal Data that is where I'd check First it might not hurt to clear your Caches in your Browser, Twitter, FaceBook any thing that uses Space like the one's I mentioned.. And it doesn't hurt to run a System Cache Clear or Even a Factory Data Reset although if you run a F.D.R you will need to Bach up any Pictures or such that you may want Saved..
One other thing make sure you have a Fully Charged Battery on your Phone any you have a Very Good Connection to either the Network or Wi-Fi..