Want to change a half switched outlet to constant power
One of the sets of hot/neutral wires are switched (probably red/white) and one is unswitched (probably black/white).  You only want one set connected to your new outlet. If you connect both, you'll short them together. Best case, your whole switched circuit will become unswitched. Worst case, you'll connect together opposite legs of your 240v feed and blow breakers or start a fire!

Try connecting the black and corresponding white to the new outlet and cap the other two wires with wire nuts. If the whole outlet ends up switched, exchanged the connected wires with the capped wires.

Edit: as Michael Karas points out (thanks), check the side of the old outlet where the white wires were connected. If that tab was not broken, go ahead and connect both white wires. The outlet may have been used as a jumper to extend the neutral to other outlets/loads. You still never want to connect both the red and black wires.