I have little hope, but will still ask: can I exchange my old TeleFinland Prepaid microSIM with a nanoSIM, keeping the same tariff? I'm really happy with my old surfing package (10 euros for 5 GB for 6 months) and I really don't want to make a nanoSIM myself.
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.
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.