This is a bug fix release which fixes a fatal error which may be seen during updates when updating interpolations, see #3022. It also fixes handling of changefiles imported via nominatim add-data.
A complete list of changes can be found in the Changelog.
Updating from any version between 3.6.x and 4.2.x to the new release is straightforward. Please follow the instructions from the migration guide. Updating from older versions is possible but not recommended. Note: when updating from 4.2.x, you need to run nominatim refresh --functions after the upgrade.