If you use Windows XP or below you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the Adobe Reader program and then click Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the Adobe Reader application and click on Uninstall.
After that, download the latest version of the software from the official web page and install it following the provided instructions. This should fix the issue.