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 in "Bash startup files".