When updating a value in a repeater you need to use you need to already know one of the other fields in the repeater on the same row.
// "field" represents the field that was changed
var field_to_update = field.closest('.acf-row').find('[data-key="key to field you wan to update"] .acf-input input');
I’m not sure what else you are doing in your code.
To trigger something that happens in a single row your action needs to be attache to each row and in addition to this it must be done on the document not on the specific field because the fields are added using JavaScript so they may not exist at the time your trigger is added.
$(document).on('change', '[data-key="key of sub field in repeater"] .acf-input input'
the last part of the selector (“input”) will need to be changed based on the field type you want the action added to.
