Project will not compile even though the same pgm used to compile and work

Projects Programming
1

Arduino: 1.8.0 (Windows 10), Board: "Arduino/Genuino Uno"

Build options changed, rebuilding all
c:\program files (x86)\arduino\hardware\tools\avr\bin../lib/gcc/avr/4.9.2/../../../../avr/bin/ar.exe: unable to rename 'core\core.a'; reason: Permission denied

exit status 1
Error compiling for board Arduino/Genuino Uno.

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.

The only change was upgrading the IDE from 1.6.13 to 1.8.0

I have no idea what is going on, pls help.

2

Some execution of a build left behind a file that can not be renamed. Delete it.