How to change the master slide in a whole presentation

I have been searching the internet, but struggle still.

I have Office 2010 as well as an existing presentation that I can use as is, but the master slide of the company has changed.

How can I insert this master slide, while keeping the presentation? I am sure there will be some formatting issues, but that is a minor. I just do not want to copy and paste more than 200 slides on the new master slide.

Thanks!

Are you talking about a PowerPoint slide presentation?

Open the presentation, click on View, then click on Slide Master, then edit away...

Yes, it is Powerpoint.

I have edited- no problem, but how to change the existing presentation so that the new masterslide/background is used?

This video from Microsoft might help:
Video: Apply one or more slide masters to a new or existing presentation - Office.com

I would help more if I could, but I'm no expert with PowerPoint