How to update a single select custom field by Rest API?

Hi together,

Is there any possibility to update a single select custom field by rest API?
Currently, I try to get the options available in the custom field, but even that I don’t get.

Searched the whole day and don’t find anything helpfull. With “/rest/api/3/field/” I find the field but not the options at all.

regards
Denis

For example, if I do that:
/rest/api/3/field/customfield_10030/contexts

I always get a

{
    "errorMessages": [
        "The custom field was not found."
    ],
    "errors": {}
}

If I try other custom fields like Company Size which seems to be a standard of JIRA, this call works. But my customfield_10030 is one, I created by myself.