Image Credit: Wikipedia
Open the Settings app on your compatible iPad.
Scroll down and select General.
On the General page, select Software Update.
On the next page, you can see Beta Updates.
Select Beta Updates, then choose iPadOS 18 Developer Beta.
Return to the General page. An update window will appear shortly.
Select the blue Update Now button, enter your device passcode, and the iPadOS 18 Developer Beta will be installed on your tablet.