How can I update my iOS 12.5.5

I'm going to guess you own an iPhone 6, which cannot be updated past iOS 12.5.5. If you need a higher iOS version for more modern apps, you'll need to retire you now out of date, no longer supported, underpowered phone in favor of a newer, more modern iPhone.

it is showing me update your phone from iOS 12.5.5 to iOS 13.4 how can I do it pls

Unfortunately, the message does not tell you that It is not possible if you have an iPhone 6.

You will need a newer phone.

marvis32 wrote:

So you mean I can’t install Facebook there is Facebook app in my phone before so I mistakenly delete it now trying to install again it is showing me update your phone from iOS 12.5.5 to iOS 13.4 how can I do it pls

That is correct. The Facebook app requires a phone with iOS 13.4 or higher. You own a phone is no longer compatible with that as it's iOS is too old and it cannot be updated further.

Time for a new iPhone as there is nothing anyone can do to help you update the iOS on your very old iPhone.

A device running iOS 12.5.5 can not be updated further.

You likely have an iPhone 6 or iPhone 6 Plus. iOS 12.5.5 is the end of the road as far as updates for the phone.