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I am amazed that the tree still looks healthy after almost 1,5 years indoors. Is it very cold in the windowsill?

Anyway, if it is a birch, I would suggest putting it outdoors as soon as the risk of frost is over. Make a slow transition from shade to full sun. Once outdoors in full sun, leave it there for all future.

Regarding your initial question, I would suggest uppotting this season and repotting next spring when buds are swelling.

Thank you all for you great help! The window sill is room temperature mostly with good sun. I will move it out to the garden when the frost risk has stopped. Is there any advice you would give to repot/up pot? When does the season start/stop sorry I have no knowledge in this field

Uppotting, taking the whole root ball out of the pot and put it in a bigger pot with some new soil around it can be done any time of the year.

Repotting, includes root work and is best done in early spring, when buds swell and are close to leaf out. The reason I say you should wait a year is that the tree has been grown in less than ideal conditions and is proboably not in full health. Never repot a tree when there is doubt about health.