2008 Ford Focus

Hi can anyone help me out to get rid of the tire low pressure on my computer dash. I had 16" rally rims my car. I took them off and put on 15" rims with my winter tires. Ever since i did the tire swap the low tire pressure message pops on my dash board. I check out the tire pressure they are all the correct pressure for the tires. So can ayone help me plz. thanks.