We have two DC in our network. Both now work in 2008 R2 mode. I want to remove the one, that was created first. Is it safe just to turn it off or should I do something to promote replica DC to be main one?
You would need to run DCPROMO, as suggested, on the DC you want to remove, but first make sure both DCs are running DNS and if this was the first DC you installed, then it probably has your FSMO roles on it - move them first. The DCPROMO procedure may move them for you, but I would consider safer and wiser to manually move them. In addition, make sure the remaining DC is also configured to be a Global Catalog (GC).
Finally, why are you getting rid of it? In general, I recommend you have two DCs per major site and two DCs on your network so that if one fails, your Active Directory isn't completely lost.