HowTo convert a database from utf8 to utf8mb4

First things you have to do is like an upgrade

Step 1 - Check requirements

The first step to upgrading Redmine is to check that you meet the requirements for the version you're about to use or install.

Step 2 - Backup

It is recommended that you backup your database and file uploads. Most upgrades are safe but it never hurts to have a backup just in case.

To understand how to back up your Redmine, please read Backing up and restoring Redmine.

Step 3 - Convert

Actually, the conversion is already done with a single command line

ALTER DATABASE redmine DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci

If you use an assistant such as phpMyAdmin, you can simply select the database there and select the item Collation under the Operations tab.

The conversion of the character set is now complete.