You can only edit a campaign if it is Inactive.
If your campaign is Active/ Paused/ Ended, it's not possible to modify or change the settings.
If you would like to change the settings for an Active campaign, you'd need to delete it and create it again.
NOTE:
If the Active campaign has already started sending SMS, you need to pause the campaign and export the unsent contacts to create a new campaign with the correct settings.
If you delete an Active/Paused campaign, the cost of all the unsent SMS in this campaign will be refunded to your account balance automatically.