I'm trying to fix the items in Skyrim that I think is way to low poly and is stuck with a .nif that I can't find anything to open it with
Here's a link to an addon:
To install it:
Download the folder
Open User Preferences (Ctrl+Alt+U) and under Add-ons, click Install from File. Then navigate to the file and select it.
It should now appear in the window and you can tick the checkbox in the upper right to enable it.
Related: Installing an addon