To update an Eloquent field dynamically using a variable for the field name, you can use the update method provided by Eloquent. Here's an example of how you can achieve this:

$model = YourModel::find($id);
$field = 'fieldName';
$value = 'updated value';

$model->update([$field => $value]);

In this example, YourModel represents your Eloquent model class. You can retrieve the model instance you want to update using the find method or any other retrieval method based on your requirements.

Next, you specify the field name in the $field variable and the updated value in the $value variable.

Finally, you use the update method on the model instance and pass an associative array with the field name as the key and the updated value as the value. In this case, since the field name is stored in the $field variable, you enclose it in square brackets [$field => $value].

This way, the field specified by the variable will be updated with the desired value.

Make sure to adjust the code based on your specific Eloquent model and field names.