If the App Store or Play Store won't offer you a 1SE update on your iPhone or Android phone, the store is usually right: it only offers versions your phone's software can run.
On an iPhone or iPad
1SE 6.3 and later need iOS 18. If your device can run iOS 18, update iOS first (Settings > General > Software Update), then open the App Store, tap your profile picture, and update 1SE from the pending list. Devices that stop at iOS 17 (the iPhone 8, iPhone X, and 2017 iPad Pro models) keep the last version that runs there, and the App Store won't offer a newer update.
On Android
1SE needs Android 11 or later. On older Android versions, the Play Store shows the last version that supports them and won't offer the current update. Check Settings > About phone and install any system update your phone offers first.
Running a very old version?
A 1SE version starting with 4 is the same app about two years behind, not a different app. Update it if your phone allows; if the store won't offer the update, one of the reasons above is why.