is it possible for me to use an update query to find things of the same name, as i would like to use it to update the "current stock" field by finding out how many movies of the same name there are finding the and takintg that number away from the "in stock" field then using that as the data for "current stock".

Current stock is a calculated value that shouldnt be stored in a table.
Instead you should store how many you have in stock, then also store who they are loaned to, then it is a simple query/calculation to get the current stock figure.