I have Euros.
I will be in Russia along the Trans-siberian railway for about 12 days. I might go to Moscow by plane or by train.
So, what is the best way to change money? I do not know how much I will spend.
In addition to previous recommendations, it is better to exchange money in Moscow since the rate will be slightly better. If you will have any spare cash at the end of your trip, you can change it back.
Avoid airport exchanges, use official (bank) exchange booths.