Every day, the trader offers 6 cards. Of those, 4 are commons and 2 are elites, all upgraded to level 2. For each card, the trader requires two unupgraded cards of the same rarity. Every 14 days, one of the commons is replaced by a legend or a unique, for this card, the trader wants two upgraded elites in return.

You can also use tokens to get these cards. All players start off with 50 tokens. Additional tokens can be aquired from the laboratory, by turing your surplus cards into tokens.
Commons will give 1 token per upgrade level, elites 2 tokens per upgrade level, legends 4 and uniques 8. Trading these tokens in at the trader for cards will require double these amounts to get cards of a similar rarity and upgrade level.