If you mean to change default directory for "Node.js command prompt", when you launch it, then (Windows case)
- go the directory where NodeJS was installed
- find file nodevars.bat
- open it with editor as administrator
change the default path in the row which looks like
if "%CD%\"=="%~dp0" cd /d "%HOMEDRIVE%%HOMEPATH%"
with your path. It could be for example
if "%CD%\"=="%~dp0" cd /d "c://MyDirectory/"
if you mean to change directory once when you launched "Node.js command prompt", then execute the following command in the Node.js command prompt:
cd c:/MyDirectory/