You can, but they need to have time to acclimate before the nights get below 15 degrees, so if it is already getting cooler nights in your location then it is likely too late to be able to move them out this year and it will need to wait until spring time when the nights get warmer again.
Outdoor guinea pigs need a lot of protection - a good, strong, secure hutch (which where possible is placed inside a shed or outbuilding), with thermal and water proof hutch covers, loads of hay and snuggle safe heat pads (day and night during winter).