First step is to paint over the Carl & Ellie text, which I mostly did with the Clone Stamp. Sample very often and do small areas at a time because the background of the mailbox has a gradient and a lot of grungy texture.
After I was done with the Clone Stamp,my background was too blotchy, so I applied a few gradients to smooth-out the transitions from light to dark. Then I used the clone stamp a bit more to bring the blotchy texture back in.
Once you've got the mailbox prepared, then select an appropriate font and apply your new text on a new layer. The font I used here is called Forte. It's not a very good match to the original font, but it was the best I had without downloading a custom font or trying to draw it myself freehand. Select the font colors from the original Carl & Ellie text (or, of course, change them if you want).
The Carl & Ellie text has a grungy, streaky quality. I simulated that very quickly using a layer mask and a brush set to low opacity, and I masked-out areas of text to let the mailbox background partially show through. This could be done better if I took my time.
Lastly, I rasterized the text layer and applied a very slight perspective adjustment (Edit>Transform>Perspective) to make the left side of the text smaller to match the perspective of the mailbox.