Hi Ralf,
If you can access the database to run SQL statements, this can be done.
Extra field parameters, like the required parameter, are saved as JSON string, so SQL can be used to modify this string and replace "required":1 with "required":0
Make sure to backup the table #__k2_extra_fields at first.
The SQL statement can be something like this, replace '#_' with your database prefix:
UPDATE #__k2_extra_fields
SET `value` = REPLACE(`value`, '"required":1', '"required":0')
WHERE id IN (Field1, Field1, ..., FieldN )
Please make sure to replace Field1, Field1, ..., FieldN with a comma separated field ids which you want to edit
