How do I gain weight in a like a week or two?

I'm 5,4 and I think I weight 103 right now I don't know maybe more. All my life I've been called skinny and little and it has always bugged me. Especially my butt. Bigger butts seem to run in my family except me. I always catch hell for it. I'm not curvy enough to wear body con or pencil dresses. I think i just need to gain weight but how? I really want to be like 120/125. But for the life of me I can't gain weight. How can I improve this skinny figure? Help?
How do I gain weight in a like a week or two?
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