Why would you need to udpate a segment created based on a list?
When you create your segment it is based off criteria. For example you may have created a segment based on subscribers who have opened your emails, or maybe clicked or maybe based on opened in a certain time frame like 30 days so you may want to keep that data in the segment updated to reflect the freshest information.
To do this login to your panel and go to the Lists area.
Once you have clicked lists you will see a view of all your lists and segments. You will notice that your segments have the word (segment) after it.
To refresh or update your list you will want to click the "update" link on the right side of your list as shown below.
Thats it. If you have any questions, please let us know.
Note: The segment is created based off a list. If the list is not mailed any longer and you are using a time frame conditon (i.e. 30 days) then after that time frame your segment will turn to 0 since there is no recent mail going to that master list. If you dont want to refresh the segment then dont click the udpate link after it is created.