Accidentally deleted message icon

Did you delete it from the phone entirely? If so, see the section in this article on restoring apps:

Delete built-in Apple apps from your iPhone, iPad, or Apple Watch - Apple Support

Or, did you just delete it from the Home screen? If so, swipe to the App Library. It's the page beyond the last Home page to the right. Tap in the search box. Find Messages in the list. Press and hold on the icon. Tap "Add to Home Screen".

Go to the App Library, which is the last page of the App Store, find it and drag it to wherever you want it.