You can do that by typing the following command in the terminal:
echo 'set completion-ignore-case On' >> ~/.inputrc
if you want to go back to the original behavior you have the open that file and remove the line.
~/.inputrc file deals with mapping the keyboard for specific situations. This file is the start-up file used by Readline, the input-related library used by Bash, perl and most other open source programs.
The configuration options in .inputrc are particularly useful for
customising the way Tab-completion works, e.g. with the ls command
When a program which uses the Readline library starts up, the init
file is read, and the key bindings are set.
You can find more information about the file syntax here.