How to migrate from iPhone 7+ to 8+

Assuming that you will use the same Apple ID from your iPhone 7+ and 8+ then : Transfer content from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

Hope this helps

Good Luck.

You Should get a question asking if you’d like to merge content from your iCloud once you have joined the account. If you have a computer with iTunes, you can back up the old device and restore onto the new.

Hello Pattathee

This Apple Article will address your concerns in a very simple and easy to understand manner : Transfer content from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support