If you want to update in place instead of migrating to a fresh server, that's possible as well. As with any major upgrade, it's highly recommended you take a snapshot of the droplet before you begin so you can roll back if problems arise.
To upgrade Ubuntu itself, run:
sudo apt-get update
sudo apt-get dist-upgrade
This command will take some time and require a reboot.
Configuration file '/etc/apache2/apache2.conf'
==> Modified (by you or by a script) since installation.
==> Package distributor has shipped an updated version.
What would you like to do about it ? Your options are:
Y or I : install the package maintainer's version
N or O : keep your currently-installed version
D : show the differences between the versions
Z : start a shell to examine the situation
The default action is to keep your current version.
*** apache2.conf (Y/I/N/O/D/Z) [default=N] ? n
answer "No" to preserve your current Apache configurations. Do the same when asked about PHP configuration files.