You may want to amend a target Segment that you have previously created. Please note however, that a Segment can only be edited if all messages it is attached to are in "Draft". If a message send is “mid-flight” (i.e. message status is Scheduled or Active) any changes you make to the target segment will NOT take effect. It will only take effect for messages that use that Segment that are in Draft status.
However, you can change the Segment name at any time.