Here’s some explicit instructions for upgrading from 2.4 to 2.6.

If for some reason you don’t have a /usr/src directory:

mkdir -p /usr/src
cd /usr/src && git init
git remote add origin git://git.dragonflybsd.org/dragonfly.git
git fetch origin
git branch DragonFly_RELEASE_2_6 origin/DragonFly_RELEASE_2_6
git checkout DragonFly_RELEASE_2_6
git pull

If you already have a /usr/src/ directory, you can just do the last 3 steps:

git branch DragonFly_RELEASE_2_6 origin/DragonFly_RELEASE_2_6
git checkout DragonFly_RELEASE_2_6
git pull

And then you can perform the normal “make buildworld…” steps outlined in /usr/src/UPDATING.