How to update 2 table data using query builder - laravel

Question

I have problem with edit and update with 2 table using query builder.
--->After i press button submit--->the data insert new row---> not update current data.
This is my function edit(only for view old data)
public function edit(Request $request, $id){
$tax_rate = TaxRate::find($id);
$tax_rate_details = TaxRateDetail::where('tax_rate_id', $id)->get();
$country = Country::all();
$geo_zones = GeoZone::all();
//dd($tax_rate);
//dd($tax_rate_details);
//dd($geo_zones);
if(!$tax_rate) {
return redirect('/');
}
return view('tax_management.edit',['country' => $country , 'tax_rate'=>$tax_rate , 'geo_zones'=>$geo_zones, 'tax_rate_details'=>$tax_rate_details]);
}
This is my update(i do validation and the saveTax is the saving part)
public function update(Request $request, $id){
$this->validate($request,[
'country_id'=> 'required',
'tax_type' => 'required',
'name' => 'required|max:100',
'code' => 'required|max:50'
]);
//dd($request->input());
DB::beginTransaction();
try{
$tax_rate = TaxRate::find($id);
$tax_rate_details = TaxRateDetail::where('tax_rate_id', $id)->get();
$this->saveTax($request, $tax_rate);
DB::commit();
return redirect()->route('tax_management.index');
} catch (\Exception $ex){
//dd($ex);
DB::rollback();
return back()->withInput()->withErrors('Fail to save');
}
}
This is my function save()
private function saveTax(Request $request, $tax_rate){
$tax_rate->country_id = $request->input('country_id');
$tax_rate->geo_zone_id = $request->input('geo_zone_id');
$tax_rate->tax_type = $request->input('tax_type');
$tax_rate->name = $request->input('name');
$tax_rate->code = $request->input('code');
$tax_rate->description = $request->input('description');
if(!empty($request->input('active'))){
$tax_rate->active =1;
} else {
$tax_rate->active =0;
}
$tax_rate->save();
if($tax_rate->tax_rate_id) {
TaxRateDetail::where('tax_rate_id', $tax_rate->tax_rate_id)->delete();
}
if($request->input('tax_rate_details')){
foreach ($request->input('tax_rate_details') as $key => $value) {
$tax_rate_detail = new TaxRateDetail();
$tax_rate_detail->tax_rate_id = $tax_rate->tax_rate_id;
$tax_rate_detail->priority = $value['priority'];
$tax_rate_detail->date_from = $value['date_from'];
$tax_rate_detail->date_to = $value['date_to'];
$tax_rate_detail->rate = $value['rate'];
$tax_rate_detail->type = $value['type'];
$tax_rate_detail->active = $value['active'];
//dd($tax_rate_detail);
$tax_rate_detail->save();
}
}
}
I want to save edit update with old(id). not create new. Please help thank you. I don't know where the code gone wrong.