If learning mode is really running, then whatever changes you make to the target temperature, by twirling the dial or pushing arrow keys in the app, will be reflected in the schedule that the Nest is building for you. Then, once the learning period is over, you can go into the schedule and modify it however you like.
Or, with learning mode not active, you can go into whatever default schedule is on the Nest and modify that.
Other than setting and modifying the schedule (which is the best way to set target temperatures, as that is how the Nest is designed), there are two ways to aim the Nest at a target of 18:
Temporary hold -- Just twirl the dial or press the arrow keys to 18. The Nest will heat to 18 until the next set point in your schedule.
Permanent hold -- Set your Eco temperature to 18 (in Settings) and then put the Nest into Eco mode.
