To install a package into MikTeX:
Close all open applications that use LaTeX/MikTeX.
MikTeX Package Manager in Admin mode. (Under the Start menu you will find it in
MikTeX → Maintenance (Admin) → Package Manager (Admin))
Type the partial or full name of the package you want to install in the toolbar and click on
In the search results, right-click on the package you want and choose
Uninstalling a package is the same as above, just choose
Uninstall in step 4.
Note: I use LEd as my primary LaTeX editor. Whenever I try to compile a
.tex file which uses a package that is not installed in my MikTeX system, LEd tries to install it automatically from the web. However, LEd's install dialog disappears while installing and neither shows the success/failure of the operation nor the progress. This is the reason I prefer to do installation/uninstallation manually using MikTeX.