What's the best way to update/refresh old posts?

For instance, say you have a post that is 2 year old already and you want to update it to the current year, how many changes should you make?

Would just changing a picture help? Or is a few sentences enough to make it seem fresh in Googles eyes? Or do you need to write quite a bit to make it seem fresh?

I ask this because one of my competitors is constantly updating their blog posts every 1 to 2 months and they are drastically rising in the ranks of Google to Page 1 with almost every one of their posts.

I can't figure out for the life of me what they are doing differently asides from updating so often.

Any tips would be greatly appreciated.