How to change some information in a shell prompt.
Add the following line to the end of your ~/.bashrc:

export PS1='\u:\W \$ '

Your prompt should now be voloda2:~ $, which is your username, a colon, the current working directory, and a $ if you are a regular user, or a # if you are root. For more prompt customization options, type man bash or visit here for a full list of options.

If you just want to change john to alex, then use export PS1='alex:\W \u \$ '. john is the name of your computer, so if you have appropriate administrator privileges open System Preferences, select Sharing and change the name to whatever you want, and your prompt will automatically update. However, if you are on a network, at work, or using a shared computer, talk to your local system admin before doing this.