See the weather forecast and dress according to the weather. Note that wind strength and cooling effect play a role in how cold it feels outside.
SMHI.se, YR.no, lavinprognoser.se
Most of the time it’s fine if you put on the clothes you have with you and don’t forget a hat and gloves. If you need to rent or borrow clothes, contact your accommodation and see what they can offer.
A small backpack that can hold a jacket is good to bring in case you need to put on or take off some clothes during the tour.