The default capabilities of the Amazon Echo with smartthings cannot at this time turn on or off AC or Heat. Because of this, Alexa will not accidentally turn either of them on or off. If you want, you can add this capability with simulated switches and CoRE or with the Ask Alexa app by @MichaelS. With Ask Alexa you give specific commands to set the thermostat to AC or Heat.
Otherwise you should be able to control these options through smartthings on your phone, though to give specifics we would need to know your thermostat model.
Using Simulated Switches
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