According to Bingo at avrfreaks.com ...
1 : Backup/Copy your <WinAVR>/avr directory to a safe place ,
or at least the "lib and include" directories within the
<WinAVR>/avr
directory as they are going to be overwritten (deleted) following
this
procedure.
2: Unzip the avr-libc-bin-1.2.6.zip somewhere , it will make an
avr-libc-bin-1.2.6 directory that contains three directories named
bin,lib
and include. The lib and include contains the new avr-libc (in lib)
,
and the corresponding header files (in include) , in bin is some
kind of
manual.
3: MAKE sure you did step 1 !!!!!!!
4: In the <WinAVR>/avr directory , delete or move the two directories
named lib
and include , so you will end up with just a bin directory.
You still need the bin directory , as that is where the avr tools
and the
compiler is located.
5: Now copy the three directories from step 2 , into the <WinAVR>/avr