Provided there is an original DVD. Current machines do not come with one.

My MBP originally came with Yosemite, but at the time it was delivered, El-Capitan was already out. So I thought, that it will be easier to update, while the machine is not customized yet.

Now I don't know, how to downgrade. The Apple Store offers only El-Capitan for download, not Yosemite.