Answers for "how to update null value in deleted_at column in laravel"

laravel migration on delete set null

$table->...->onDelete('set null');

get deleted value laravel

You can get also soft deleted record using withTrashed() of Laravel 
Eloquent. It will return all record from table.

Item::withTrashed()->get();

You can get only soft deleted row using onlyTrashed() of Laravel Eloquent.

Item::onlyTrashed()->get();