Call to undefined method save() in laravel update code - development

Call to undefined method save() in laravel update code

In my code am trying to update the information but I receive error as

Call to undefined method IlluminateDatabaseQueryBuilder::save()

$user_id is not a primary key for preferences table. Its just a column name

Controller code for update:

public function update(Request $request, preferences $preferences,$user_id)
{
 $preferences = preferences::where('user_id', $user_id);

  // $preferences = DB::table('preferences')->where('preferences.user_id','=', $user_id)->first();

  $preferences->detergent= request()->detergent;
  $preferences->fabricsoftener= request()->fabricsoftener;
  $preferences->starch= request()->starch;
  $preferences->save();
    return redirect()->route('user.home.preferences', [Auth::user()->id]);
}

View code in blade file looks like

<form class="form-group coupon-form" action="" method="post">
                  
                  <div class="col-md-12">

                          <div class="heading-title-alt text-left heading-border-bottom">
                                  <h4 class="text-uppercase">Cleaning preferences</h4>
                          </div>

                          @foreach($preferences as $preferences)
                          <div class="form-group">
                              <label>Detergent</label>
                              <select name="detergent" class="form-control">
                                  <option name="detergent" value="none" @if ($preferences->detergent == "none") selected @else "" @endif > None</option>
                                  <option name="detergent" value="scented" @if ($preferences->detergent == "scented") selected @else "" @endif >Scented</option>
                                  <option name="detergent" value="hypoallergenic" @if ($preferences->detergent == "hypoallergenic") selected @else "" @endif>Hypo-Allergenic</option>
                              </select>
                          </div>

                          <div class="form-group">
                              <label>Fabric Softener</label>
                              <select name="fabricsoftener" class="form-control">
                                  <option name="fabricsoftener" value="yes" @if ($preferences->fabricsoftener == "yes") selected @else "" @endif>Yes</option>
                                  <option name="fabricsoftener" value="no" @if ($preferences->fabricsoftener == "no") selected @else "" @endif>No</option>
                              </select>
                          </div>

                          <div class="form-group">
                              <label>Starch (Laundered & Press items only)</label>
                              <select name="starch" class="form-control">
                                  <option name="starch" value="none" @if ($preferences->starch == "none") selected @else "" @endif>None</option>
                                  <option name="starch" value="light" @if ($preferences->starch == "light") selected @else "" @endif>Light</option>
                                  <option name="starch" value="medium" @if ($preferences->starch == "medium") selected @else "" @endif>Medium</option>
                                  <option name="starch" value="heavy" @if ($preferences->starch == "heavy") selected @else "" @endif>Heavy</option>
                              </select>
                          </div>
                          @endforeach

                          <button class="btn btn-small btn-dark-solid full-width  " id="login-form-submit" name="login-form-submit" value="login">Update</button>
                  </div>
                </form>

Please help me understad the error and solve it.

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