Hi @miraperry,
The first step would be to clean up the gap and remove whatever damaged material is there, so you have a clean surface to fill. This can be done using tools such as a utility knife, paint scraper and sandpaper, ensuring you wear appropriate personal protective equipment such as gloves and long protective clothing.
Once the gap has been tidied up and any loose material has been removed, you would use a flexible gap sealant such as No More Gaps to fill the seam before painting over the top.
Check out How To Silicone a Gap for some guidance on the process of installing sealants using a caulking gun.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
