There are unfortunately not any bulk operations available in Riak, so this has to be done by updating each object individually. If your updates however arrive in bulks, it may be worthwhile revisiting your data model. If you can de-normalise your products, perhaps by storing a range of products in a single object, it might be possible to reduce the number of updates that need to be performed by grouping them, thereby reducing the load on the cluster.
When modelling data in Riak you usually need to look at access and query patterns in addition to the structure of the data, and make sure that the model supports all types of queries and latency requirements. This quite often means de-normalising your model by either grouping or duplicating data in order to ensure that updates and queries can be performed as efficiently as possible, ideally through direct K/V access.