Got a letter from irs

I have got a letter from IRS which is proposing for change to tax or credits on my tax return for 2018. I filed the form for 2018 from hr block and  it showed i have to receive a refund of $1199. Unfortunately i finally received this letter at the end of september. I worked for 2 companies in 2018. I changed the company in December so the second company had last month w2 and the first sent me till November. However when i checked my wages transcript on IRS it shows the wages from my 2nd company only. I have no idea , how to exactly respond to the request.