Solution
Locate the dam's data; go to:
[1. input/edit] - [1.1. Sows] - [1.1.5. Mating/farrowing/weaning data (individual)]
Press 'F3' (Search), enter the dam's animal number, and press 'Enter'.
Locate the correct cycle data.
Go to the 'F-9 Individual animals' tab.
Locate the animal in question and modify the tattoo details.
Click the green checkmark to confirm.