If a meeting request is declined, the meeting is not saved to your calendar and the meeting request message is moved to the Deleted Items folder.
There is no way to show a declined meeting on your calendar. You can only show accepted or tentative meetings on your calendar. To request this feature, go to Outlook UserVoice.
Accept a previously declined meeting
If you previously declined a meeting and want to change your response, you can find the original meeting request in the Deleted Items folder.
Switch to your Deleted Items folder. You can find your Deleted Items folder in the Folder Pane.
If your Folder Pane isn't visible, select View > Folder Pane > Normal.
Double-click the original meeting request in the Deleted Items folder. Then click Accept or Tentative.
If the Deleted Items folder has been emptied, this option isn't available. Ask the meeting organizer to forward you the original meeting request.