You can put degus outside if temperature is not going under 15 (+-) degrees of Celsius most of the days. Although they'd probably manage even lower temperatures. But every degu is different, so watch their reactions and their condition in first days. Some can manage cold very well and some not. Also give them some nest box with plenty of material like napkins, so that they can get warm inside if the weather will get colder.

Much more dangerous are high temperatures and mainly direct sunlight. Put their cage only in shadows, not too big part of the cage should be on sunlight. Degus can die after being only 1 hour on sunlight!