Cannot upgrade as later version has been installed

There are three things that you can check if the system is saying that a newer version of the add-on is already installed:

1. Go to your Control Panel > Programs and Features and see if the application is showing there. If so remove it.
2. Open the file ADDONSLOCALREGISTRATION.SBO in the SAP Business One folder and remove the script that is similar to the script below:
   <AddOn Exe="Enprise_JobCosting.exe" Installer="EnpriseJobCostingInstaller.exe" InstallerMD5="CA6D94F186B5D798E93DDDAACE83E006" Name="Enprise Job Costing" Path="C:\Program Files\SAP\SAP Business One\AddOns\Enprise Solutions\Enprise Job Costing" SelfUpgrd="false" Space="Enprise Solutions" UID="Enprise Solutions_Enprise Job Costing" UnInstEstTime="60" UnInstParams="/x" UnInstaler="EnpriseJobCostingInstaller.exe" UnInstallerMD5="CA6D94F186B5D798E93DDDAACE83E006" Ver="8.81.109"><Common ID="12" Name="WIN08_8\SQL08"/></AddOn>
3. Inside the table ENPRISE_SECURITY, reset the field VERSION back to 999. This will be for the row with the APPLICATION value being 501.
   That should remove any reference to the existing version of the application and then allow you to install the new version.