Your monitor? Did you mean the DK2? I assume you're talking about a 60Hz laptop display and a 75Hz DK2. When you run DK2 in 'direct' or 'extend' mode, your laptop will remain 60Hz, and the DK2 will get 75Hz. There are bugs with the headset that sometimes confuse this in games when in extend mode - resulting in the DK2 only getting 60Hz. This is usually resolved by making the DK2 the primary display.
As for locking your games to 75fps, this happens automatically if you're at 75Hz with v-sync on, which is the default for most DK2 demos. If v-sync is off, you'll have an unlocked framerate, which is usually fine too. Some prefer to run with v-sync off and add a 75fps limiter, this can be done with Nvidia Inspector (if your laptop has Nvidia hardware) and 'radeonpro' with AMD... I think. What's your laptop spec?