Can Help me How to instal iOS after i already download, because I have a problem.

Hello Ekayolanda! 

Thanks for reaching out to the Apple Support Communities. I understand that you’re having trouble installing the iOS 12.4.2 update on your iPhone. Not to worry!

The article below will walk you through how to remove and download the update again (at the bottom of the article). Once you’ve done that see if you’re able to install. If you receive another error, follow the rest of the troubleshooting in the article.

Get help with over-the-air iOS updates

I hope this is helpful.