How to update a recordset that is not updatable

I have a query based on four tables, Customers,Orders,Order Details and Products.

I want to update all the records in the field "Liters" from the table "Order Details" If the field

"Cartons" from the table "Order Details" is 0(ZERO), by multiplying the field "Quantity "from the

table "Order details" to the field "Size" from the Table "Products".
I have built a Form and tried to update the recordset but i receive the message that the recordset

is not updatable.How could i update my records?
In a nutshell, the updating should read as follows
liters[Order Details] = size[Products]*[quantity[Order Details]

I need this updating to mend all the errors for the whole year.
I will be grateful for any help