If I target .net 4.5 but publish to a machine that has .net 4.0, how to make the application to run?

I has a web application that test ok when published to local IIS on Window 8. Then I publish the web app to a server machine that has .net 4.0. I get the error message: Parser Error Message: Unrecognized attribute "targetFramework" Note that attribute names are case-sensitive. Source Error: <compilation targetFramework="4.5"/><httpRuntime targetFramework="4.5" /> Source File: D:\Domains\xxxxxxxx.com\wwwrootweb.config Version Information: Microsoft.NET Framework version 2.0.50727.7026.

Then I log in to the server machine, make the following changes in Web.config file. <compilation targetFramework="4.0" /><httpRuntime targetFramework="4.0" /> Now my web site is down even I put the file App_offline.htm in the root directory. How can I fix this problem ? Thank you in advance.