To merge several calendar data into one calendar, switch the Calendar to the list view and:
- drag the items to the destination folder,
- or use the Move to folder command.
To do that, perform the following steps:
- Select the calendar you want to remove and switch to the list view. In Outlook 2010/2013/2016/2019, navigate to View > Change View > List
- Click on one item, then press Ctrl+A to select all
- Use the Move to folder (or Copy to folder) option, or drag the items to the folder to which you want to move them, or use Ctrl+X and Ctrl+V (cut and paste).
Note: Contacts folders can be merged the same way; the only difference is that you will need to use the Phone list view.