I guess you mean how do you delete old data from splunk. There are three methods of achieving this.
To delete EVERYTHING in an index, you need to use the clean command from the command line.
splunk clean eventdata -index <
see Remove indexed data from Splunk for more detail
To delete specific events, you need to add the "can_delete" roll to your account, search for the data to delete and then pipe to the delete command. For example from the search bar of your browser to delete < mydata> older than a year :-
see Delete command for more detail
To automatically delete data in an index older than an age, you need to set the frozenTimePeriodInSecs attribute in indexes.conf. eg. to set the default to delete data after 1 year use :-
frozenTimePeriodInSecs = 31536000
see Set a retirement and archiving policy for more detail
as an aside, the delete command doesn't actually delete the logs, it jut makes them unsearchable. So if you're looking to save some disk space this might not be the best method to use.