Common causes
- The network cannot reach the update server;
- The portable or binary folder is not writable;
- Security software blocks the updater.
Fixes
- Desktop: download the latest installer again and install over it — chats are untouched;
- Linux binary: extract the new version into the same folder;
- Mobile: update through the app store or the official APK.
Rolling back
Telegram publishes archives of older builds, so you can install a previous version over the current one. Very old builds may fail to connect, so check compatibility first.