I have 5s 16 GB and downloaded ios 11.2 but it is failed to install popping up an error occurred. What to do now

You could not have enough space on your device (you need 7-8GB free). Read this support article:

Get help with over-the-air iOS updates - Apple Support

If nothing helps, try to update via iTunes (latest version 12.7.2). If you don't have a computer, borrow on from your friends.

If you can't do it, go to the nearest Apple Store and they will help you.

Regards

Giulio

Get help with over-the-air iOS updates - Apple Support

Hi

You may not have enough space to update you need to delete some apps / photos.

16G is not very good now upgrade iPhone when you can.

Cheers

Brian 🇬-1F1E7;