Hi and welcome to Telia Yhteisö!

I believe you can exchange your old SIM card to a new nanoSIM at Telia Shop or Telia Express stand if you have the old SIM-card frame with you, and an ID card. Please, check this thread: https://yhteiso.telia.fi/t5/Puhelimet-tabletit-ja-muut/Prepaid-SIM-kortin-vaihtaminen/td-p/170184   , it's in Finnish but google translate will do.

Briefly: Visit Telia Shop with your excisting SIM-card and the package/sim card frame. Please take your ID card with you. There should be spare sim cards for prepaid users. If you want to confirm if the store has spare cards, you may always contact the customer service.

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If you have lost the original package/sim card frame containing the info (serial number, PUK/PIN codes etc.) options are dire. The only option is try to cut micro-SIM to a nano-sim. If you decide to do so, you should acknowledge that SIM card might get damaged. That isn't an impossible task, but there are risks. Your device can be damaged if the card isn't correct sized or shaped. Card can also be stuck in a device's sim slot if it has any notches etc.  

Thanks a lot! It was easier than I expected. I found that card with a PUK code :)

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@lizazim@  kirjoitti:

Thanks a lot! It was easier than I expected. I found that card with a PUK code :)

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No problems. :smileyhappy:  So you were able to change the sim card at Telia shop/express point and things are ok?