Hi Artem,
It is not possible to update bidding at item level. However, you could use AdGroupCriterionService.mutate operation to update bidding at product group level via Google Ads API if you have the resource name of the AdGroup criterion to be modified. You could specify the resource name of the listing/product group in the AdGroupCriterion resource and set the 'bid_modifier' value to update the bidding. Let us know if you need additional information.
Regards,
Nikisha Patel, Google Ads API Team