T key on keyboard has come loose. How do I replace it?

@roish most likely time to replace the key cap. Here is the guide for the Key Cap Replacement One of the crucial things to keep in mind is that Apple makes it clear that "Although keycaps can be replaced, the butterfly and switch housing cannot. A damaged switch housing or butterfly requires replacement of the entire top case." You also want to read through this wiki MacBook Keyboard Not Working for more on the diagnostic and replacement. To determine the proper keypad part number, you will have to identify your computer a little bit better. Choose the right one from here MacBook Pro 13" Retina Display or use your serial number to identify your MBP. The Apple part numbers differ based on the model you have
Depending on your geographic location, keycaps are available at places like this and many more.