This function changes the name of the current homepage and renames a page to Home.aspx. It does of course assume the current home page is Home.aspx. The welcome page URL is case sensitive. See https://jcallaghan.com/2019/06/switch-between-modern-sharepoint-homepages-using-pnp-powershell/ or more information. #Website

Set-SPNewHomepage.ps1

<#
.SYNOPSIS
    Easily replace the current homepage with an alternative page you have created.
.DESCRIPTION
    This function changes the name of the current homepage and renames a page to Home.aspx. 
    The script currently only works when the homepage is /sitepages/home.aspx. 
    It does of course assume the current homepage is Home.aspx. The welcome page URL is case sensitive.
    A switch to remove the previous homepage rather than rename it is also included.
.EXAMPLE
    Set-NewHomepage -siteurl https://demo.sharepoint.com/sites/intranet -pagerelativeurl /sitepages/newhome1.aspx -removeprevious:$false
.NOTES
    Author:     James Callaghan (@jamescallaghan)
    Website:    www.jcallaghan.com 
    Date:       08/06/2019
#>
function Set-SPNewHomepage {
    [cmdletbinding()]
    Param (
        [Parameter(Position=0,Mandatory=$true,
        HelpMessage = "Provide the url to the SharePoint site where the pages are located.")]
        [string]$siteurl,  
        [Parameter(Position=1,Mandatory=$true,
        HelpMessage = "Provide the relative url to the page that should become the homagepage i.e. /sites/abc/xyz.aspx")]
        [string]$pagerelativeurl,
        [Parameter(Mandatory=$false,
        HelpMessage = "Confirm if you wish to remove the previous homepage.")]
        [bool]$removeprevious

    )
    
    Process {
        Clear-Host
        # Pnp Connect
        write-host "Connecting to site $($siteurl)..."
        Connect-PnPOnline -Url $siteurl -UseWebLogin

        # Get web to get the relative url
        $web = Get-PnpWeb
        $currenthome = Get-PnPHomePage
        Write-Host "The current homepage is $($currenthome)."
        write-host "Renaming $($pagerelativeurl) to make it the new homepage..."

        # Stop if the homepage is not as the expected home.aspx.
        # This could be updated to chage the welcome page property.
        if($currenthome -ne "SitePages/Home.aspx"){
            write-host "Error homepage not not as expected." -ForegroundColor Red
            break
        }

        # Rename the page if remove previous is false
        if($removeprevious -eq $false){

            # Create a unique text string to avoid duplicate page names
            $now = Get-Date
            $nowstring = $now.ToString("yyyyMMddHHmmss")

            # Check proposed home page exists
            $baseurl = $($web.url).Split("/")
            $pageExists = Get-PnPFile -url $("https://" + $baseurl[2]  + $pagerelativeurl) -ErrorAction SilentlyContinue
            if($null -eq $pageExists){
                    write-host "Error page not found." -ForegroundColor Red
                    break
            }

            # Rename existing home page
            Rename-PnPFile -ServerRelativeUrl $($web.ServerRelativeUrl + "/sitepages/Home.aspx") -TargetFileName $("oldhome" + $nowstring + ".aspx") -Force
        }
        else
        {
            # Remove the previous homepage but recycle it
            Remove-PnpFile -ServerRelativeUrl $($web.ServerRelativeUrl + "/sitepages/Home.aspx") -Recycle -Force
        }

        # Rename page being promoted as the home page
        Rename-PnPFile -ServerRelativeUrl $pagerelativeurl -TargetFileName "Home.aspx" -Force 
        write-host "New homepage set successfully." -ForegroundColor Green

        # Disconnect
        Disconnect-PnPOnline
    }
}

.\Set-SPNewHomepage.ps1 -siteurl "https://demo.sharepoint.com/sites/contosoweb/" -pagerelativeurl "/sites/contosoweb/SitePages/NewHome1.aspx" -removeprevious:$false