How do I revert to a previous iOS version on iPhone 13 Pro Max?

No, you cannot revert to iOS 18.

However, there is basic troubleshooting that might help. First is to restart the phone and check all apps are also up to date.

Hi, I’m sorry, Apple does not currently and never has allowed a downgrade of Operating System versions.

iOS 26 and Liquid Glass are simply the design Apple has chosen to go with, for now. The iPhone User Interface has had several major redesigns in its near 20 year history. The last major redesign was iOS 7, back in 2014. We all somehow managed to get through them all. 

You may want to review the information in the article I linked to below. It explains the adjustments you can make to mitigate the effects of Liquid Glass that you’re experiencing. 

Liquid Glass - Love It Or Hate It - It's … - Apple Community

Apple’s installed user base is over 1.5 billion users. Making a single user interface perfectly compatible for every user is literally impossible. Apple will make changes (or not make changes) based partially on the feedback they receive through their official channel, linked below and good luck.

Product Feedback - Apple

No.

I have a 13 Pro Max

If you check Settings > Battery > Battery Health & Charging......

What is the current Maximum Capacity of the battery?