更改根元素 xml python etree

Question

I have a xml that start like this:

<rdf:RDF xmlns:cc="alfa" xmlns:cim="beta" xmlns:entsoe="gamma" xmlns:md="delta" xmlns:rdf="epsilon">

cim:stuff

</cim:stuff>

</rdf:RDF>

With the following code, I get as root the last atribute

tree = ET.parse(r'case.xml')

root = tree.getroot()

root

out:

<Element '{epsilon}RDF' at 0x00000221959E2F40>

I want to change the root and set beta

How I can do that? How _setroot(element) works?