If it's the same generation i.e.2.x then simply installing it will over right the older version.You can download the latest version 2.5 from the Adobe web site. Provided you have a valid licence it will update automatically
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However if it's running slow I suspect as been mentioned earlier the problem may be with the Database. You should try optisiming this first. From Lightroom menu, select Catalog settings. You'll get a pop up, at the bottom, select optimise and relaunch. See if that makes any difference