When you finish approving time sheets and that the pay period is ready to be exported, it's recommended to finalize 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, Finalized)
To do so, you can simply click on the link (Edit) found under the dates of the pay period. See the following picture :
If you want to modify a pay period that is already finalized, see this article!