vi is a text-only text editor that's available on most systems including YDL. For a more detailed tutorial, see here:
http://www.eng.hawaii.edu/Tutor/vi.html
Mini vi tutorial:
Start with: vi /path/to/your/file
for example: vi /etc/yaboot.conf
Use the arrow keys to move around
Use the x key to delete text at the current position
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Press the i key for Insert mode
Type your changes
Press the Esc key to exit Insert mode
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Press :wq to save your changes and exit
or
Type :q! to exit without saving changes