Add a black border in photoshop

I have a very high res image - a png file. I would like you to add a black border in photoshop so that the image size increases from it's current size - 294 x 86 (ish) to 340 x 120
That's cm.
I want you to write a message explaining exactly how to do this in the latest version of photoshop .
It's really important that the image remains the same size and a png. No compression, no data loss.
Thanks
Laura