1994 Nissan Sentra safety switch

Transmission problem
1994 Nissan Sentra 4 cyl Two Wheel Drive Automatic 60000+ miles

Hello. I'm going to try to change out the saftey switch on my mama's 94 Sentra. It wouldn't turn over for her at church the other day, so she got it jumped off and made it home. She changed out the battery, but the car still won't turn over. She has power (lights, radio, dash) so I figure it's the safety switch. Two questions: where is it located? And how do I change it out? Thank you so much.