Firstly, check that your device can install iOS 12. You didn't state which iPad you have which makes it impossible to give you more exact assistance. Your iPad may be capable of upgrading beyond iOS 12. See this page.
Then follow the instructions to update your iPad as described on this page.
Firstly, check that your device can install iOS 12. You didn't state which iPad you have which makes it impossible to give you more exact assistance. Your iPad may be capable of upgrading beyond iOS 12. See this page.
Then follow the instructions to update your iPad as described on this page.