That's rather simple: just stop filling the helium tank and the nitrogen tank. The system will warm up by itself in about 3 days. You can follow the warming up by checking the two thermometers (VTI-bottom and λ-plate, T1 and T2). 7Ω is nitrogen temperature, 19Ω about 150K, 28Ω is 300K.

In any case there are some very important measures to be taken while warming up, to avoid explosions of tubes or capillaries due to the expansion of gases (Helium, Nitrogen and air) when warming up from low T to room T. Never forget these steps before warming up, or cryostat damage can be a consequence.

1. Close the needle valves.
2. Connect He-tank to recovery (should be already like that).
3. Connect VTI to recovery.
4. Conenct λ-plate pumping line to recovery.
5. Put an overpressure release after the speedy valve of the vacuum interspace. When the cryostat is still a bit cold (~19Ω or less), open the speedy valve. The underpressure will keep the space sealed automatically, but if there builds up overpressure, the overpressure release will go off.

Warnings and advises:

1. NEVER NEVER NEVER vent the superinsulation, neither to air nor to Helium, nor to nothing.
2. If you do not want to keep the cryostat warm for long, it's convenient to keep a Helium vessel connected to the recovery close to the cryostat, so that the boiloff will keep the cryostat “conditioned” at Helium athmosphere.
3. If you want to keep the cryostat warm for a long time, avoid to vent any space to air.