I am at the moment evaluating Filemaker Pro for my company and have 2 tables. One is for customers and one is for devices. Devices are assigned to  a customer and the tables have a relationship via the customer. I wish to be able to transfer the customer details from the customer table to the device table via a update button but  am floundering on setting up the script for the update button

Let's start at the beginning. A customer can have many devices, correct? Therefore, the relationship between the two would be Customer ->Device by  Customer::id = Device::id_customer.

To show customer info on a layout based on the Device table occurrence, you'd place the related customer fields on the layout. You do not copy customer info into the device table.

To get a bit more complex, you may wish to maintain the history of device assignments. For that, you'd need a third table that stores the combinations of customer and device. So, Customer->CustDevice<-Device. This middle table is called a "join table." In it you may have the fields that record data about the "assignment" of that device to that customer (date_assigned, date_returned, etc.)