Updating lookup values
If you change data in the match field of the related table or in the lookup source field for a lookup, FileMaker Pro doesn't automatically update the data in the lookup destination field. To update the data, you need to look it up again.
Important  Relookup Field Contents updates all fields looked up by the match field across all records in your current found set. You can't undo your actions when you update lookup values.
To update lookup values:
Note  The Relookup Field Contents and Replace Field Contents commands no longer commit data when they are performed. If your solution depends on the data being committed, you must explicitly commit the data before performing those commands. For example, if you modify a field value and then try to perform a relookup on that field without committing the data, the relookup search will be based on the old data rather than the modified data. (For more information about committing data, see Committing data in records.)
Tip  You can update lookup values automatically by defining a button to perform the Relookup Field Contents command, or by defining a script that uses the Relookup Field Contents script step.
Related topics 
About lookups
Defining lookups
About relationships
Defining a button