Hi Sebastian,
Renaming an instance should be just a matter of:
seq uninstall -n TheOldName
mv C:\ProgramData\Seq\Instance\TheOldName C:\ProgramData\Seq\Instance\TheNewName
seq install -n TheNewName
(Forgive my typos above :))
Listing instances is only possible from the Windows Services control panel currently, though you can do (in PowerShell):
Get-Service | where { $_.Name -like 'Seq*'}
Hope this helps!
Nick