First thing is to move your placeholder (i.e., the manually entered) event to the same start time as the Byga sourced event). You can do this by dragging the event or clicking on it and changing the start time. Make sure the manually entered event type (game or practice) is the same as the system event you would like merged.
Then you go to the team's directory on the top menu bar and find your team. The menu on the right of your team will have a merge events option.
Byga will search your future events and find duplicates (same event type and same start time). It will present those and you can select the ones that it can merge for you. In rare cases the merge is not possible (e.g., multiple Byga sourced events with the same start time).
The latest availability response from each player is preserved. And any event notes are preserved.