You may have already noticed that site navigation style have changed on newly created SharePoint Online communication sites. Experience is currently different on sites created earlier.

Megamenu navigation enabled
Old style navigation

Here is a quick-n-dirty PowerShell script to change experience back to old style navigation on newly created communication sites.

[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client") | Out-Null
[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client.Runtime") | Out-Null

$webUrl = "<site url>"
$username = "<username>@<tenant>"
$pw = read-host -Prompt "Enter password" -AsSecureString

$ctx= New-Object Microsoft.SharePoint.Client.ClientContext($webURL)
$ctx.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username,$pw)
$ctx.Web.MegaMenuEnabled = $false //Disables megamenu experience

Edited 22.8.2018 17.33:

Of course you can do it with PnP PowerShell.

Connect-PnPOnline -Url <yoursite>
$web = Get-PnPWeb
$web.MegaMenuEnabled = $false //$false to disable, $true to enable