Apex Class:

public with sharing class ExampleController {

    @AuraEnabled(cacheable=true)
    public static List < Account > fetchAccounts() {

        return [ SELECT Id, Name, Industry, ( SELECT Id, FirstName, LastName FROM Contacts ) FROM Account LIMIT 10 ];
      
    }
 
}

LWC HTML:

<template>
    <lightning-card>
        <div class="slds-m-bottom_small">
            <lightning-button
                label="Collapse All"
                onclick={clickToCollapseAll}>
            </lightning-button>
            <lightning-button
                label="Expand All"
                onclick={clickToExpandAll}>
            </lightning-button>          
        </div>      
        <lightning-tree-grid
            columns = {gridColumns}
            data = {gridData}
            key-field = "Id"
            hide-checkbox-column = true
            onrowaction={handleRowAction}>
        </lightning-tree-grid>
        <p slot="footer">
            <lightning-button
                label="Get Updated Data"
                onclick={fetchUpdatedData}>
            </lightning-button>
        </p>
    </lightning-card>  
</template>

LWC JS:

import { LightningElement, track, wire } from 'lwc';
import { NavigationMixin } from 'lightning/navigation';
import fetchAccounts from '@salesforce/apex/ExampleController.fetchAccounts';
import { refreshApex } from '@salesforce/apex';

export default class TreeGrid extends NavigationMixin( LightningElement ) {

    gridData;
    wireData;
    gridColumns = [{
        type: 'text',
        fieldName: 'Name',
        label: 'Name'
    },
    {
        type: 'text',
        fieldName: 'Industry',
        label: 'Industry'
    },
    {
        type: 'text',
        fieldName: 'FirstName',
        label: 'FirstName'
    },
    {
        type: 'text',
        fieldName: 'LastName',
        label: 'LastName'
    },
    {
        type: 'button',
        typeAttributes: {
            label: 'View'
        }
    }];    

    @wire(fetchAccounts)
    accountTreeData( value ) {

        console.log( 'Inside wire' );
        this.wireData = value;     
        const { data, error } = value;

        if ( data ) {
            
            let tempData = JSON.parse( JSON.stringify( data ) );
            console.log( 'Data is ' + JSON.stringify( tempData ) );
            /*let tempjson = JSON.parse( JSON.stringify( data ).split( 'Contacts' ).join( '_children' ) );
            console.log( 'Temp JSON is ' + tempjson );*/
            for ( let i = 0; i < tempData.length; i++ ) {
    
                tempData[ i ]._children = tempData[ i ][ 'Contacts' ];
                delete tempData[ i ].Contacts;
    
            }
            this.gridData = tempData;

        } else if ( error ) {
         
            if ( Array.isArray( error.body ) )
                console.log( 'Error is ' + error.body.map( e => e.message ).join( ', ' ) );
            else if ( typeof error.body.message === 'string' )
                console.log( 'Error is ' + error.body.message );

        }

    }

    clickToExpandAll( e ) {
        const grid =  this.template.querySelector( 'lightning-tree-grid' );
        grid.expandAll();
    }

    clickToCollapseAll( e ) {

        const grid =  this.template.querySelector( 'lightning-tree-grid' );
        grid.collapseAll();
     
    }

    handleRowAction( event ) {
       
        const row = event.detail.row;
        this[NavigationMixin.Navigate]({
            type: 'standard__recordPage',
            attributes: {
                recordId: row.Id,
                actionName: 'view'
            }
        });

    }

    fetchUpdatedData() {

        refreshApex( this.wireData );

    }

}

js-meta.xml:

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>52.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__Tab</target>
    </targets>
</LightningComponentBundle>

Output:

After clicking “Get Updated Data” button:

Test Acc 45 is changed to Testing Acc 45.

Contact first name and last names are also updated in the tree grid.