Did you notice metallic flakes and/or burning smell from the fluid? If so than that would indicate internal failure in the tranny somewhere. If it's just dark then it's dirty and needs replaced, it may or may not be the cause of slipping. To replace it you would have to disconnect a hose from the tranny and let it drain out of it by starting engine and put in Neutral. After that, remove the drain plug from bottom of transaxle case to get the rest to drain.
This guide will help walk you through the steps
https://www.2carpros.com/articles/how-to-service-an-automatic-transmission
Fluid Types
AUTOMATIC TRANSAXLE
LUBRICANTS
A/T fluid .................... DIAMOND ATF SP III or equivalent
Capacities
AUTOMATIC TRANSAXLE
LUBRICANTS
A/T fluid L (qt) .................... 7.8 (8.2)
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Wednesday, June 9th, 2010 AT 8:47 PM