The job description says the salary is mid 40's, but I'd like at LEAST 50k. How can I negotiate for this?

When I applied I was required to put my salary requirements (I could not say negotiable or 0) and because the salary said mid 40's I didn't want to leave the range in case they filtered me out so I said 47k - 49k. After doing research about living costs and the average salary for similar roles in the area, I found that its about 55k. I would like to have at least 50k. How do I negotiate for this given what I originally wrote on my application.

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