Follow this steps below.
Login to your blogger dashboard and then the blog you always use => click on posts
Now you will see all you post that you have composed so select that post you want to change the date and click Edit
Now on the left hand side of the page you will see settings click on it.
Move down to the published tab and click on it.
You have to click on post date and time and then change the date to what ever date you want it to be.
Lastly click on the update button at the top right hand corner of that page and you are good to go.
Do forget to drop your comments below thanks.