A 64 bit executable of UPX in Version 3.08 Dev have been released next to a traditional 32bit version.
Homepage and program description
If you are not so familiar with command line parameter tools, there exists a great GUI which supports almost all switches and parameters from UPX: Free UPX 1.4
Support for the 64bit version can be implement if you replace the upx.exe in Free_UPX folder with the 64bit one.
Compressing 64 bit windows executables is not supported yet. It will show an error message "can't pack new-exe."
Downloads UPX 3.08 Dev: