How can I update my iPhone 5s

If you have an iPhone 5S as stated in the title, you can update it to iOS 12.5.7. If you have an iPhone 5, as stated in your description, it can be updated to iOS 10.3.4

You can check your current version here:

Settings → General → About

You can download and install software updates here:

Settings → General → Software Update

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

Your iPhone is old, it cannot be updated to the latest software. This may help:

Update your iPhone or iPad - Apple Support