You can use Marketo to update the value of a field. For this, you will use the Change Data Value flow action.
You can also block a field from being updated. See Block Updates to a Field for details.
1. Find and select the field you want to change the value of.
2. Enter the value you want, and you're done!
You can also use tokens in New Value.
You can enter 'NULL' (no quotes, all caps) in New Value to clear the field. See Clear Field Values for details.