How to switch git user at terminal?

I am trying to push a project to a remote repository from the command line.

From inside my local directory, I hit:

$ git push

and obtain the following error:

remote: Permission to username1/repo.git denied to username2.
fatal: unable to access 'https://github.com/username1/repo.git/':
The requested URL returned error: 403

Where username1 is my github account username hosting the repository I want to push to and username2 is an old account I used to use on this machine.

I am using OS X Yosemite (v10.10.5) on a Macbook Air. And I would prefer to use https instead of ssh.

How do I update to username1 so I can successfully push to my remote?

Edit: To be clear, I am not talking about simply editing the config user object, e.g.,

$ git config --global user.name "Billy Everyteen"
$ git config --global user.email "[email protected]"

They have nothing to do with authentication. My question deals with user authentication necessary to write to my remote repository.

In addition to changing username and email from terminal using git config:

$ git config --global user.name "Bob"
$ git config --global user.email "[email protected]"

you'll need to remove authorization info from Keychain. This is something I've also struggled with until I found that I also had certificate in my Keychain.

Open up Keychain access, click on All Items and search for git. You will get some items like this:

Delete them. Now try to push the repo and git will ask you to write password for the user and you will be good to go.

For cli users, just use this : git config credential.username 'Billy Everytee'