My battery is dead. After jumping my car and driving around last night for about and our and a half the car is still dead this morning. The battery retained no charge. I'm about to go with a friend now to buy a new battery. Someone mentioned finding one with 900CCA so I will do that but I will need to know how to replace the old one safely. Can someone just tell me how to change the battery safely? thanks.

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W126 1983 300SD 286,000 miles and ticking
Baby blue exterior Grey MB tex
Recent work:
Replaced air cleaner mounting brackets and heat shields
Replaced alternator, fan and power steering belts
Replaced positive battery terminal
Replaced negative battery terminal and cord
New Duralast Battery

My car needs work.