The official migration guide is available in the official documentation.
Please be aware that this is still experimental and problems might be present. You
should read the documentation to its fullest before you attempt migration.
You can also check Gitlab issue about migration to get an idea about features that are covered by migration.
If you are going to perform migration on Turris 1.0 and Turris 1.1, be aware that Turris OS 5.0 is not supported on internal storage. You have to migrate to SD card first, you can follow this tutorial.