If you updated your project on another computer and only want to download the new changes to your current computer. You don't have to clone it again, you can just click pull.
If you changed any files of your existing project, you can just click Push instead of creating a new local repository.
If you are trying to upload a unity project "with" Unity currently opened. It will not let you. This is because when Unity is opened, your project folder will temporarily have a UnityLock file. To solve this, Close both Unity and Visual Studio and try again.