For updating a sub field, you could use update_sub_field(...) method. Here is the link: update_sub_field()
And creating a repeater field programmatically is possible. Export the PHP code. From your wp-admin, go to Custom fields > import/export. To update the data for the repeater field, use update_field(...) method. Make sure to use the $field_key for the reference. Have a look at the docs for more info: update_field()
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