Once you have the latest source code, look at /usr/src/UPDATING. This file lists, in reverse chronological order, any warnings and special notices about changes to FreeBSD that are of special interest to people who build from source. This file tells you if you must take any special actions before rebuilding your system or if any major system functionality has changed. If you want your system to work after the upgrade, follow those instructions exactly. Also examine the new GENERIC or NOTES kernel configuration files for any new options or interesting kernel changes.