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:

  1. 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

    List View in Outlook
  2. Click on one item, then press Ctrl+A to select all
  3. 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).
    Move, Copy or Drag the Items 

Note: Contacts folders can be merged the same way; the only difference is that you will need to use the Phone list view.