GIMP :: How To Modify And Delete Text On Picture

I would like to modify a text on a picture for a friend. Let me explain : here is the picture I'd like to modify :

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I'd like to delete the facebook thing and replace it. But I don't know how to do it. I thought I would select and delete but the result is terrible because the color is not uniform, take a look at this :

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The top of the sheet is okay, the rest is not...

Second question. How to make a text looks "real".

I have thought : let's take a picture of a someone holding a blank paper and add a message.

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It's not bad but it doesn't look as if I had stamped the message. So I was wondering is there any tool to do so ? Or to make the text fit the picture. As you can see, the quality is not perfect, the first thing coming in my mind is "pixels". It seems we have two different kind of "pixelisation" (I don't know if it's the correct term for what I want to say) : an "old" picture with a new text...I tried the pixel filter it's not working fine.