I've had two Herman tortoises since they were weeks old. They are now 6yrs old, have always lived in a vivarium and have never been hibernated. 

I've tried putting them out in the garden in summer when its really warm but they just look for cover and try to bury themselves to sleep. Even moving their vivarium to a different room in the house that is a little cooler and even with the light and heat lamp on they are reluctant to wake up.

Is it too late to transition them to living outdoors in the summer and hibernating them in winter?

I can't continue to keep them indoors permanently and am thinking of rehoming but want to prepare them for summer outdoor living.

Looking for best advise 

The reason they look for cover when put out is that you've put them out when it's hot & sunny. They have no way of cooling down except moving to shade.

Click on "The Tortoise / " box & download a Caresheet. Study it carefully & continue to learn. 

Tom