First go to Products

Apply filters or select the applicable product records you want to move to another client and then click Move, e.g.

In the next screen, find and select the client record where you want to move the filtered/selected product records to and then click Move

Finally, if the client you moved the products from no longer has any products, you can safely remove the client record. You have thereby effectively merged 2 client records by first moving the product records of the source client to the target client and then subsequently removing the source client.