Open a terminal, run
# If ~./inputrc doesn't exist yet, first include the original /etc/inputrc so we don't override it
if [ ! -a ~/.inputrc ]; then echo '$include /etc/inputrc' > ~/.inputrc; fi
# Add option to ~/.inputrc to enable case-insensitive tab completion
echo 'set completion-ignore-case On' >> ~/.inputrc
Start a new shell / terminal.
to make this change for all users, edit
For details, see
man bash . Yes it is a long page, but bash is a somewhat complex program, and if you want just search that page for "case-insensitive" to go to the relevant section. People usually learn bash one option at a time or one bash script at a time and it takes a long time to master all the nuances. Your interest may vary.