To change to an absolutely specified directory, use the entire name, starting with a backslash \\\\, thus:

cd /home/username/project/abc

If you want to change to a directory near your current on, you can specify a relative location. For example, if you are already in /home/username/project, you can enter the subdirectory abc thus:

cd abc

If you want to go to the directory above the current directory, you can use the alias ... For example, if you were in /home/username/project/abc and wanted to go to /home/username/project, then you would do the following:

cd ..

This may also be called going “up” a directory.