When you create a Design Component, you have the option to allow the instances to change the text or images through the Framer UI.
To enable Text or Image overrides, you have to enable them in the "Master" Design Component. By default, all text or images in the Design Component are exposed, but not enabled.
To enable them, you need to make sure that the checkbox next to the text or image property is selected.
After these are selected, any "Instance" of this Design Component will have the option to change or override these properties right from the Properties Panel.
Pro tip: You can rename the "props" of the master design component as you enable them. It's always good practice to use descriptive and meaningful names to make it easier to understand at first glance!