I'm afraid it is not clear to me what you are asking. It seems that you are generating your template instances within the repeater from a dataset, and that your template includes a dropdown. Are you wanting to make changes to the template repeater dataset using a template instance's dropdown from within the repeater?
If so, the key is identifying how many layers up you must go to get to the template repeater. It's usually about five layers up, so your template event handler will have to call it in the following manner: event.source.parent.parent.parent.parent.parent.
This post will guide you on how the templates are nested within the template repeater: