help me updates my phone

You must be at least on iOS 14 to download the app. If you have an iPhone 6S Plus as stated in your description, you can update to iOS 15.7.3, and that way you can download and use the app.

You can download and install software updates here:

Settings → General → Software Update

Remember to connect to Wi-Fi and a charger. Your iPhone must have more than 50% power to update without a charger connected. If you are on a software change, it may be necessary to do the update from a computer. See my links below.

You can find more useful information here:

Update your iPhone or iPad - Apple Support

If you want to update your iPhone via a PC or Mac, see here:

• Mac: Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

• PC (iTunes): Update your iPhone, iPad, or iPod touch with iTunes - Apple Support