You can do this using the Update Multiple Rows feature.
First, you must test this on one stock item to ensure you have the results as you wish.
Go to the stock items in question – Inventory / stock items / find – and then select all and right click on the results.
Select Update Multiple Rows as below:
Select ManufacturerID as the field to update.
Enter that ID number in the Change Value To... box.
Click Apply.
See below:
You will need to have the ID of the manufacturer yo u wish to change to.
You get this from their supplier / manufacturer records as below: