to add tags to an existing document, use a Builder that works at a specific point in a document to remove tags from a document, find the tags and simply remove them.

the following snippet

doc = Nokogiri::XML(communication)
Nokogiri::XML::Builder.with(doc.at('root')) do |xml|
  xml.credit("value" => @cost)
end
doc.search('//document/data').each do |node|
  node.remove
end
doc.to_s

modifies this

<communication>
 <document><data>xxx</data></document>
</communication>

into

<communication>
 <document></document>
 <credit value="12"/>
</communication>