how can I do the cellular update

See this Apple article: Manually update your carrier settings on your iPhone or iPad

https://support.apple.com/en-us/HT201270

You don't need to tap anything.

If there’s an available carrier settings update for your iPhone, a pop-up will appear on the screen asking if you want to update. 

After 15-30 seconds pass and no pop-up appears on your iPhone, that means you don't need to update or there aren’t new carrier settings updates for your iPhone.

I must confess, I've never done carrier settings update for my iPhones.