Why there is no option for an update?

If there is no option to update your iPad when plugged into a PC/Mac or when you go into the settings, that means one of two things, your iPad is up to do date and running latest iOS, or is running the latest iOS the iPad can handle

Not all iPads can be updated to iOS 13.

If you give us the info about your iPad and what iOS it is running we could then tell you what the last iOS is for the device

Writing an effective Apple Support Commun… - Apple Community

What year and model iPad do you own and what iOS version is it currently running?

What is the “A” model number of your iPad?

The “A” model number of your iPad can be found on the lower area of the aluminium back of your iPad in small print letters right underneath the large, bold printed” iPad”.