Try to remove it from Installed Updates:
1. Click on “Control Panel” in “Start menu”
2. Go to “Uninstall a program” in “Control Panel”
3. Click on “View installed updates” in “Programs and Features”
4. Right click on Update which reads Powershell and uninstall it.
Or
Open "Computer" and navigate into your Local Disk C: (or wherever your windows install is located).
Navigate into the Windows folder and then the System32 folder.
(C:\Windows\System32\)
Note: If you are running a 64-bit version of windows, it is also listed in the SysWow64 folder.
(C:\Windows\SysWow64\)
Find the "WindowsPowerShell" folder. Right-click on it and click Rename.
Rename the folder to WindowsPowerShell2.
Close the window and attempt to perform the upgrade again.
You will still have the shortcuts for Powershell in your start menu and
anywhere else they appear, you can right-click them and delete them.