The correct phrase is "back to basics." It means returning to fundamental principles or essentials. "Back to basis" is not a commonly used phrase in English.

Back to your job because there isn't any other options left.

Back to you old place (house, office, state, town, ect.).

Back to your spouse after you've failed in a relationship.

Back to your vechile.

A back workout routine has the potential to increase your overall comfort and improve your posture. A back workout routine that equally strengthens the major muscle groups in the back can lessen your chances of back injury and can decrease back pain in your daily life. A back routine should work the upper, middle and lower areas of back, working primarily to strengthen the muscles before being focused on muscle bulk.

back pannel connector

Diagram of a system unit(back view)