I’ll copy this here, just in case:
Upgrading to 15.1
We’re starting our 15.1 builds with some devices (listed below), with others migrating to 15.1 sooner or later. Note that several older devices are unable to receive official builds at this time due to the lack of support for working HAL1 camera recorder, which was broken by the treble changes, but don’t worry - we’re working on it.
From official LineageOS 14.1
- (Optional) Make a backup of your important data
- Download the build either from download portal or built in updater app
- You can export the downloaded package to the sdcard from the Updater app by long-pressing it and then selecting “Export” on the top menu
- Download proper addons packages (GApps, su…) for Oreo 8.1 (8.0 packages are not compatible).
- Make sure your recovery and firmware are up to date
- Boot into recovery mode
- Format your system partition
- Install the LineageOS 15.1 build, then install all your addons WITHOUT rebooting
- When you’re done, reboot the device.
Note that:
- Wiping data is not needed when upgrading from an official build
- While you’ll be able to download the update from the updater app, you won’t be allowed to install a 15.1 build on the top of a 14.1 one automatically
From anything else (including unofficial LineageOS builds)
- (Optional) Make a backup of your important data
- Download the build from download portal
- Download proper addons packages (GApps, su…) for Oreo 8.1 (8.0 packages are not compatible)
- Make sure your recovery and firmware are up to date
- Boot into recovery mode and wipe data
- Install the LineageOS 15.1 build, then install all your addons without rebooting
- When you’re done, reboot the device
Note that wipe data is always needed when coming from something that’s not an official build, regardless of the Android version
Source: Lineage OS blog, Changelog 16 - Smart Styles, Treble is trouble and Omfg Oreo – LineageOS – LineageOS Android Distribution