To manually change a members join date or to backdate their join date first you need to find the member. There are two ways of doing this.
First, if you just made the sale then go to "Opportunities" tab and select "Sales" the most recent sale will be at the top. The second way is to use the "Search" option (top left corner). Type the name of the member you are looking for. Note: You need to refine the search by "Sale" and not by "Members".
Click on the person's name that you are looking for and under "Lead" you can change the join date to the desired one.
Note: You will need to be logged in as a Manager or Administrator to be able to change the date.