If you are nervous about using Terminal forcommand a you may understand, not I concur with Bjorn. It is a means of resetting the menu.
Resetting the 'Open With' menu will remove duplicates and ghost applications (ones you have deleted) from the list. You reset the 'Open With' menu by rebuilding the Launch Services database your Mac maintains. There are multiple ways to rebuild the Launch Services database, including third-party system utilities like Cocktail and Onxy.
If you don't own a system utility that can rebuild the Launch Services database, don't worry; you can perform the rebuild yourself using the Terminal command set outlined in Bjorn's post..