You may have to take a stem cutting and start over if the stem is mushy or soft at the base. I think your plant has gotten too much water. A soil moisture meter is inexpensive and very useful when caring for plants that need damp but not wet soil. A smaller pot would also make overwatering less likely. But don’t transplant it now because it is looking stressed.
https://www.gardeningknowhow.com/houseplants/dumb-cane/growing-dumbcane-dieffenbachia.htm

https://miraclegro.com/en-us/indoor-gardening/how-to-grow-dieffenbachia.html#:~:text=Dieffenbachia%20grows%20best%20with%20soil,plant%20needs%20to%20be%20watered.


https://www.bhg.com/gardening/plant-dictionary/houseplant/dieffenbachia/