In Windows, this is done at:

Person or Family tab of People Workspace

First - make a backup of your file.

Main Menu bar > Edit
Manage Facts
Select "Baptism or Christening" from pick list

Select "Data Options" button on right panel
Select "All" or pick selected persons
(If you select All, that is everyone in your file)
Select from choices at bottom:

xx Change Selected Fact to Different Fact - Choose Birth