Changing field names
When you change a field name, FileMaker Pro changes the name on each layout and any field references used in calculation or summary fields. The field label changes on layouts if it matches the original field name. See Adding and changing field labels.
To change a field name:
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With the database open, choose File menu > Manage > Database.
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For Field Name, type or paste the new name.
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Click Change.
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Note  FileMaker Pro uses the order in which fields appear in the Fields tab of the Manage Database dialog box for all dialog boxes that list fields. To define a custom order, drag the double arrow Double arrow icon next to the field definition to move the field up or down in the list. To reorder by field name or type, click the Field Name or Type column heading.
To change a field name in Table View:
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Related topics 
About naming fields
Using a reserved word or symbol for a field or table name
Adding and changing field labels