pipelines update parent record from child record

hello,

I have two tables. 

parent named location has a field serial number
child named equipment also has a field new serial number 

they are related by parent - Record #ID with a text reference proxy  "location ID" to the child.

the trigger is set so when i create / change a child record and i fill in "set" a new serial number I would like the parent serial number updated to serial number in the child..

I have tried many options

 I can't seem to find out how to push the new serial number from the child to the serial number field in the parent table. 

thx

-N