Error:Execution failed for task ':app:transformClassesWithInstantRunSlicerForDebug'. > java.io.IOException:

:app:transformClassesWithInstantRunSlicerForDebug'. java.io.IOException: Failed to delete

-- try to clean project and rebuild project.

From @Kael: A clean is performed via Build -> Clean Project or Build -> Rebuild Project.

Kindly follow these steps: Goto

1>File
2>Settings
3>Build,Execution,Deployment 
4>Instant Run
5>Uncheck (Enable Instant Run to hot swap code)

So this helped me.

In my situation Build Menu > Clean Project and Build Menu > Rebuild did NOT work. Instead I had to delete the build folder manually from the filesystem (using Finder on OSX, Explorer on Win).