You can rename a tag with the Set-Tag cmdlet.

Get-Tag -Name 'external' | Set-Tag -Name 'NewName'

Thanks,

but how to rename vm tag-Assinged tag for multiple vm via script.

how to rename vm tag- Assigned tag for multiple vm specific vm list.

Read the list and then use a loop over all the VMs.

I mean can we import excel sheet with number of vm list to rename vm tag.

Yes, you can.
If it is a CSV you can use Import-Csv.
If it is an XLSX you can use the Import-Excel cmdlet from the ImportExcel module.

can you help me with this script import vm in csv format and then rename tag.

So in the end you don't want to know how to code this, you just want a script.
Seen your past history where you don't seem to reply to threads you started, I'm going to pass on that.