Palm rolls. It depends a bit on what kind of Synthetic Dreads you wear, but palm-rolling your Dreads between your hands or between your fingers can make your Dreads more flexible.
Make as many updos as possible. When your dreads have just been installed, it is valuable to make many different updos. Twisting, braiding or bundling your dreads will also make your dreads smoother. Make a ponytail, half a ponytail, make beautiful buns or wear them in a braid. By using your Dreads a lot in different updos, your Dreads will become a lot less stiff.
And you can immediately try which updos you like which is a win win!