Visit this Solr wiki pages for more information:
https://cwiki.apache.org/confluence/display/solr/UpdateXmlMessages

Here's the structure of the update operation:

<add commitWithin="">
    <doc boost="">
        <field name="" boost="" update="set|add|inc">

Optional attributes for the "add" element:

• commitWithin: (milliseconds) time for the document to be committed.
  
  

Optional attributes for the "doc" element:

• boost: (default: 1.0) a float number for the document boost value.
  
  

Optional attributes for the "field" element:

• boost: (default: 1.0) a float number for the field boost value.
  
  
• update: "set" | "add" | "inc"
  
  With:
  
  • set: sets a value (if null remove the value, or, in the case of a multi-valued field, remove all the values).
    
    
  • add: adds a new value to a multi-valued field.
    
    
  • inc: increments a value by a specific number.
    
    
  Important: If none of the document "field" elements sets the "update" attribute, then the operation will be considered an "add" operation.
  Which means that the document will be added if it's a new document, or updated if it was already added (missing fields will be set to null or to a default value).
  

Example: (Make sure that the fields "id", "field1", and "field2" are defined in your SchemaXml).

<add>
    <doc>
        <field name="id">1</field>
        <field name="field1" update="set">field1 1-1</field>
        <field name="field1" update="set">field1 1-2</field>
        <field name="dynamicField1_s" update="set">dynamicField1_s 1-1</field>
    </doc>

    <doc>
        <field name="id">2</field>
        <field name="field2" update="set">field2 2-1</field>
        <field name="dynamicField2_s" update="set">dynamicField2_s 2-1</field>
    </doc>

    <doc>
        <field name="id">3</field>
        <field name="field1" update="add">field1 3-1</field>
        <field name="field1" update="add">field1 3-2</field>
        <field name="field2" update="set">field2 3-1</field>
    </doc>
</add>

Here's an example of an http request:

$ curl http://localhost:8983/solr/COLLECTION-NAME/update?commit=true \
-H "Content-Type: text/xml" \
--data-binary '<add><doc><field name="id">1</field><field name="data_s" update="set">beta</field></doc></add>'

Here's an example of an http request that uses the parameter stream.body:

http://localhost:8983/solr/COLLECTION-NAME/update?update.contentType=text/xml&stream.body=<add><doc><field name="id">1</field><field name="data_s" update="set">beta</field></doc></add>

In order for this request to work, you need to enable Streaming in "solrconfig.xml":

<requestParsers enableStreamBody="true" />

If you want to update the "solrconfig.xml" file directly in the Configset, make sure to reload all collections that use the updated Configset.
Visit this page for more details: http://lucene.apache.org/solr/guide/8_5/requestdispatcher-in-solrconfig.html