When you finish approving time sheets and that the pay period is ready to be exported, it's recommended to finalise the pay period for two reasons;
To restrict the pay period modification from individuals
To protect the dates included in the pay period if you ever change the frequency of your future pay periods
Note : Only an Administrator can modify the status of a pay period (Open, Closed, Finalised)
To do so, you can simply click on the link found next to the dates of the pay period. See the following video :
If you want to modify a pay period that is already finalised, see this article!