button to update a field only when another field value = X

That button will only update a single record.  Mass updating will require a different approach which is likely beyond the scope of what I can answer on this forum in detail.

If you want to update all records, then you would need to use a saved table to table import and run that via an API_RunImport or else use an Action to update all the records. 

 You may want to contact me off line for one on one assistance if you are lookingfor a mass update.

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