I am changing my seasonal tire to winter tires. My seasonal tires are size 205/60r16. Is it okay to change them with 205/55r16 sized tires?
Yes. It will have a slightly lower sidewall height, but should work fine.
Just for completeness:
For a 205/55r16 tire, the numbers mean:
205 - width of the tire in mm. On some wheels, you can go 5-10mm (approximately) over without a problem. However, it's good practice to stay with the exact with of the stock tire
55 - height of the sidewall as a percentage of the width. (AKA, aspect ratio.)
16 - diameter of the wheel in inches.
If your width (205mm) and diameter (16") match the tires that came on the car, you won't have fitment issues. However, a lower profile (sidewall height) tire may lead to a rougher ride.
See also Understanding Tire Sizes