You can update to 0.10rc3 like this if you want to keep the previous binary around. I prefer to keep a few back as a safety net:
cd ~
git clone -b v0.10.0rc3
https://github.com/bitcoin/bitcoin.git bitcoin0.10rc3
cd bitcoin0.10rc3
./autogen.sh
./configure --with-incompatible-bdb
make
make check
(I'd been running 0.10rc2 since there was no need to update to rc3, but just downloaded and compiled rc3 this way).
(You may not need the --with-incompatible-bdb )
It all went through fine, but now when I start bitcoind it still reports its running 0.9 .
How do I make it start 0.10 ?