How to update, change, modify pi_flexform values and fields in tt_content

UpdatePiFlexformInTtContent.sql

UPDATE tt_content
SET pi_flexform = REPLACE(
    pi_flexform, 
    CONCAT(
        '<field index="settings.classForTable">', 
        CHAR(10), 
        '                    <value index="vDEF">HpTableST1</value>',
        CHAR(10), 
        '                </field>'
    ),
    '<field index="settings.cssClassForPlugin">
                    <value index="vDEF">HpTableST1</value>
                </field>
                <field index="settings.cssClassForTableSection">
                    <value index="vDEF">row</value>
                </field>
                <field index="settings.cssClassForTable">
                    <value index="vDEF"></value>
                </field>'
)
WHERE list_type='stahlcranesystems_tableview';

You should definitely go for UpdateXML instead. 😉
BTW: I lately published a XML query gist: https://gist.github.com/vertexvaar/0cb83e53217bd1b477a80c1f42cc09fa and https://xmltoolbox.appspot.com/xpath_generator.html will help you find the correct xpath