The way to add the transition peak back would be to choose the peak again.
In this case where there is only one candidate peak to choose from, you would probably need to remove the peak for all of the transitions, and then choose it again.
You could do this by unchecking and then checking again the checkbox in the "Chosen" column in the "View > Live Reports > Candidate Peaks" window.

If you want to undo all of your manual peak adjustments for everything in the Skyline document, and get back to the peaks that Skyline chose immediately after chromatograms extraction, you can use the "Refine > Reintegrate" button.
In the Reintegrate dialog, be sure to check the "Overwrite" manual integration checkbox. Then, when you OK that dialog, Skyline will put all the peaks back where they used to be.

In your screenshot, I would not expect the ratio to change very much when you removed a transition peak because the removed peak had such a small area.
When you remove a light transition peak, Skyline also removes the corresponding heavy transition peak so that the number of terms in the numerator and denominator of the peak area ratio calculation remains the same.
The light to heavy peak area ratio is calculated by dividing the sum of the light transition areas by the sum of the heavy transition areas. Removing a small number from either or both of those sums has only a small effect on the resulting ratio.

If you want to see more significant digits of the ratio you can right-click on the column in the Document Grid and choose "Number format" and then choose "Full Precision" in the "choose format" dropdown.
If you saw all of the decimal digits of the ratio you would probably see a different number when you removed even a small peak.

If you want to see full precision on all of the numbers you can do "File > Export > Report".
In the Export Report dialog if you choose "Invariant" in the "language" dropdown you will get a report which is easier for a computer program to read which will have full precision numbers and no spaces in the column names.
-- Nick
-- Nick

Hi Nick
I sincerely appreciate your help.
You let me know everything I needed in a very easy way.

Thanks,
Hongsang