Upgrade windows with iso files?

Can I mean with a windows 10 iso file, originally. Win 7 laptop currently linux, upgrade laptop?

differently when i insert win 7 iso file do i need my key (which i no longer have) is there no free legal version just to have windows on my laptop again?

With windows you always need a key for activation

There's no free legal version, except Linux.

Yes you need a key. E.g. https://www.rakuten.de/produkt/microsoft-windows-10-professional-vollversion-3011811840?cid=adwords&sclid=a_rmm_de_pla_804-543-6946_3011811840_c&key-sofort_147280

You need the ISO to create a bootable USB stick or burn a bootable DVD.

https://forum.golem.de/kommentare/applikationen/windows-10-microsoft-preist-windows-10-update-mit-riesenhinweis-an/was-passiert-eigentlich-wenn-man-windows-ohne-lizenz-installiert/102011,4533651,4533651,read.html

It just runs as if it were a normal licensed version. Only the information about missing licensing appears and the settings for adapting the optics (background etc.) are restricted, which can be avoided.

In theory, you could use an installation without a license as normal. In a way, however, this is illegal (especially in the business environment).

That's right but you don't have to activate Windows 10 (until now)