use Illuminate\Database\Eloquent\Model;

 class User extends Model
 {
-    public function getFirstNameAttribute($value)
+    protected function firstName(): \Illuminate\Database\Eloquent\Casts\Attribute
     {
-        return ucfirst($value);
-    }
-
-    public function setFirstNameAttribute($value)
-    {
-        $this->attributes['first_name'] = strtolower($value);
-        $this->attributes['first_name_upper'] = strtoupper($value);
+        return \Illuminate\Database\Eloquent\Casts\Attribute::make(get: function ($value) {
+            return ucfirst($value);
+        }, set: function ($value) {
+            return ['first_name' => strtolower($value), 'first_name_upper' => strtoupper($value)];
+        });
     }
 }