You don't need a function. It can be done with an update query. Have you carefully considered the advice that Pat Hartman gave about not storing calculated values? If so, and you have determined that combining your two fields is the best scenario for you, drag the field to be updated into the grid of an update query and put the expression to combine the fields in the UPDATE TO: line.
Make sure the size of the field to be updated is adequate to contain all the characters needed.
If the target field is a primary key, the query will fail if duplicates are encountered.