1. Delete the Battle.net tools folder.

Here's how to do that:

1. Restart the computer.
2. Ensure that show hidden files and folders is enabled.

3. For Windows Vista and 7: Click Start and select Computer.

-Windows XP: Click Start and select My Computer.
-Windows 8: Windows Key + Q and select Computer.

4. For Windows Vista and 7: Double-click the C drive, then the ProgramData folder, and then delete the Battle.net folder.

-Windows XP: Double-click the C drive, then Documents and Settings, then All Users, then Application Data, and then delete the Battle.net folder.
-Windows 8: Double-click the C drive, then the ProgramData folder, and then delete the Battle.net folder.

Note: The above steps may cause Battle.net to forget game install locations. If your games can’t be found, you will see Install on the Battle.net game tab instead of Play. To find your installation, click Locate this game or run the game launcher.

2. Delete the Battle.net Cache folder.

Here's a support article with steps on how to do that:

Deleting the Battle.net Cache Folder (PC)

3. Uninstall and reinstall the Battle.net Desktop app.

Here's a support article with steps on how to do that: Uninstalling Battle.net Desktop Application

4. Changing the "Install" button back to the "Play" button.

After reinstalling the Battle.net Desktop app, you might see an "Install" button where the "Play" button used to be. This is happening because the Battle.net Desktop app does not recognize where the game folder is currently installed.

In order to resolve this:

-Click on the blue Battle.net icon located on the top left hand corner of the Battle.net Desktop app.
-Select "Settings".
-Select "Game Install/Update" from the left hand side of the Settings window.
-Click on the "Scan for Games" button.

That should start an automatic search for any Blizzard games currently installed on your system. If any games were missed upon completion of the scan, please click on the "Locate" button and manually select the game folder's location.