ios 26 is clunky, laggy has a major battery drain issue

It is not possible to downgrade, and never has been with any version.

• When did you upgrade and was it iOS 26.0.1? You should expect battery drain, lag, and heat for a short period of time after any update.

About Apple software updates - Apple Support

• What is hour Battery Health at Settings > Battery > Battery Health > Maximum Capacity? The iPhone 13 is likely due for a Battery replacement if the health is below 80%, and that would have been true whether it was updated or not.
• Post the screenshot of Settings > Battery, of your green battery graph for the previous 24 hours along with the top apps using the battery. Below that you see system usage and if Home & Lock Screen is still consuming a large portion of your battery, then that is an indication that indexing of files is still occurring after the update.
• And finally, the next update is currently being beta tested, and we cannot comment on the issues that have been resolved. It is a violation of the terms here and also the user agreement for being a tester. Since you are unable to downgrade, you may find that when that version is officially released, it may resolve some issues you have noticed.

Sorry, but there are no methods by which Apple supports downgrading the iOS on an iPhone.