How can I update custom order fields in the frontend I am struggling with using additional custom order fields. (Added in the Craft Commerce settings at "order fields", in the field layout manager)

I try to update them like this in the checkout process:

&lt;form method="POST"&gt;
    &lt;input type="hidden" name="action" value="commerce/cart/updateCart"&gt;
    &lt;input type="text" name="myCustomField" value="Test123"&gt;
    ...
    &lt;input type="submit" value="Update Custom Field"&gt;
&lt;/form&gt;


But in the Order in the CP, the field stays empty. Do I have to use another name for the field, like fields[myCustomField] or whatever?

I can update them in the CP, but not from the front-end...
Correct, you need to use fields[customFieldName] as the input name.

The following form should do it:

&lt;form method="POST"&gt;
    &lt;input type="hidden" name="action" value="commerce/cart/updateCart"&gt;
    &lt;input type="text" name="fields[myCustomField]" value="Test123"&gt;
    ...
    &lt;input type="submit" value="Update Custom Field"&gt;
&lt;/form&gt;