The first step is to unpack the new release archive into a new directory. Do not try to overwrite an existing installation. The 2nd step is a full copy of the Atmel/ directory from your existing installation into the new tree. This copy has to include the Appnotes*/ directories and the avrasm32.exe file from Atmel. These files are verbatim copies from an Atmel AVR Studio installation and are not included into the AmForth distribution (guess why).