Are you replacing existing batteries with the same type of battery? If so, it should be a straight-forward swap: make careful note of the existing connections before removing the old ones, then make the same connections when putting the new ones in.
If not replacing existing batteries, we're going to need a
lot more information...
In either case,
ALWAYS disconnect the negative lead
first, the the one that runs from the batteries to the chassis, and reconnect that lead
last. That's the safest procedure. If have that negative lead connected, and are wrenching on a positive lead, accidentally touching the wrench to any exposed chassis metal could cause a short with hundreds of amps of current. That can cause sparks, burns, and in the worst case it can weld the wrench in place until the wrench melts or the batteries explode.
Safety first: don't wrench on any other connections while that ground connection is in place.
!
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