1. Select the desired UI Button
2. Scroll down the inspector and click the “Add Component” button
3. In the list of components that will appear, select “Event” and then “Event Trigger”
4. You will notice that a new “Event Trigger” area appears in the inspector for the button
5. There, click on the “Add new even type” button
6. A list of even types will pop-up. Choose “PointerEnter”
7. Next, on the “PointerEnter” sub-area that will appear within the “Event Trigger” area in the inspector (when your button is selected, of course), click on ‘+’ to add a new item to the list of events
8. add the function

https://gamedev.stackexchange.com/questions/116801/how-to-display-text-when-mouse-hover-button-in-unity-3d