Receiving access denied error from Visual Studio when trying to change target framework

It sounds like either:

1)The file has been marked read only. Check this by viewing the properties of the csproj.

Or

2)Your user does not has sufficient permissions to change the file - check security on the csproj.

I have found that if you get the error, hit save, close visual studio and reopen the project you will find that 86.5% of the time the setting was changed.