Active OpenBSD development is known as the -current branch. These sources are frequently compiled into releases known as snapshots.
Aggressive changes are sometimes pushed in this branch, and complications can arise when building the latest code or upgrading from a previous point in time. Some of the steps for getting over these hurdles are explained on this page. Make sure you've read and understand how to build the system from source before using -current and the instructions below.
In general, it's far easier to use snapshots, as developers will have gone through much of the trouble for you already.
You should always use a snapshot as the starting point for running
This process typically consists of downloading (and verifying) the appropriate
bsd.rd file from the
directory of your preferred mirror, booting from it,
(U)pgrade at the prompt.
Any installed packages should then be
upgraded after booting into the new system.
Upgrading to -current by compiling your own source code is discouraged for everyone except for experts, as difficult build-time crossing-points can occur often, and no assistance will be provided. In case of failure, use a snapshot to recover.
Most of these changes will have to be performed as root.