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Please use and update https://unicode-org.github.io/icu/processes/release/tasks/standards.html instead.
The text below will be deleted after the next release.
Make sure the timezone (Olson) data is up to date.
Olson timezone data should be regularly updated in the trunk. If the data is out of date, follow the instruction described in the readme file.
Once the timezone resource file is generated by the process above, the file should be checked into zoneinfo64.txt in the trunk.
Update Unicode Data
Update script code enums
Update script code enums with new codes from ISO 15924, send a notice for them as public API (born @stable) to the icu-design list.
Starting with ICU 55, add script code enum constants only after assignment of Unicode script property value aliases; otherwise the names of the constants may become confusing.
Soon after a Unicode release, work to assign property value aliases for scripts that are known to be added in the next version, then update ICU.
Constants for script codes like Latf and Aran that are not subject to separate encoding may be added at any time.
See details in icu/trunk/source/data/unidata/changes.txt and keep the list of script codes not yet in ICU up to date there.
For new codes see http://www.unicode.org/iso15924/codechanges.html (check for recent changes with the code highlighted in the left column and "Add" in the right column) and http://www.iana.org/assignments/language-subtag-registry (Search for "Type: script" and look for recent "Added:" dates.) Discuss what to do if these two sources are out of sync.
Sync ICU with CLDR and tag CLDR.
Update ICU data from CLDR, tag CLDR. Instructions here.
Update MeasureUnit Code
ISO 4217 numeric codes
Make sure the ISO 4217 numeric code mapping data is up to date.
See the instruction in https://github.com/unicode-org/icu/blob/master/tools/currency/readme.txt