Transmission not working properly

That is a sign of a couple items. The seals and pistons in the apply section are worn and the clutches are worn. The issue is that when cold the parts are a bit loose but the cold fluid is thicker and they still work. Once the transmission starts to warm up the parts can flex and the fluid thins out and now the transmission starts to slip from the lowered pressures and clamping forces. At this point a rebuild or replacement would likely be the best solution. A fluid change might help but unless the transmission has been regularly serviced new clean fluid could actually remove some of the material that is helping the transmission to operate at all.

This guide shows many of the steps to service that transmission but even if it helps it will be very short lasting as the worn parts are still there:

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission