R2's head is made of hard plastic (styrene) and will NOT pop out. So you'll need to disassemble the droid to swap heads. Let me warn you, not all the R2s are built the same! The '95 Artoo and the Hologram R2 are built similarly, while some others (tools R2 and booster R2) are put together differently. Disassembly for each one uses the same steps, but you may not
be able to.mix-and-match the parts.

Step 1

Heat the R2 in hot water for 1-2 minutes. When you heat the legs they
get a little soft. Carfully pop them off the studs.

Step 2

Then take an exacto knife and split the droid up the seams on both
sides.
Use a very sharp knife and try to do this as exactly as possible so your
astromech doesn't have "scars" on his seams.

Step 3

The head will come loose when the torso is split. (The R2 with rocket
booster's head is one piece with a stem that extends all the way down into
the torso. '95 R2 and Hologram R2's heads are connected to the torso by a
small lip. I am not sure about R2 w/ saber and w/ pop-up scope.)