It looks like you have an automatic transmission.
First you need to make sure you follow the manufacturer’s recommendation as a start.
I strongly recommend replacing the fluid at least every 30 or 40,000, even if the manufacturer list something longer.
Do not every wait until there is evidence of a problem. Transmissions like to hide their needs until it is too late. It is like going to the doctor. You want to see your doctor once or twice a year, just to make sure.
Doing so has saved my life. That little spot that did not look right, sent me to the doctor. The next day I was on the operating table with the possibility of waking up with only one leg. Getting to the doctor early saved my leg (over 40 years now) That transmission is the guy who put off seeing his doctor, or did not want to pay for the office call.