Стоит ли обновлять iPhone 13 Pro Max до iOS 26?

There are millions upon millions of people who have updated an iPhone 13 to iOS 26. If you'd like the newest feature and security updates, then yes, update to iOS 26 (currently iOS 26.6). Expect some extra heat and your phone to slow down for several days after updating. It will reindex the millions upon millions of bytes of data on your phone in preparation for iOS 27 which will be coming in a few weeks.

Do make sure your battery is not depleted by opening Settings > Battery > Battery Health - if it shows 80% or less, the battery in your has used it's useful life and is need of replacement. The biggest issue people have with iPhone from about 5 years ago is a battery whose health is poor.