Change map marker icon to custom icon

Hi,

We have a map embedded on the below page and we would like to change the map marker icon.

https://aplancareers.wpengine.com/vacancies/locations/

How can we achieve this? We have a small PNG file ready to upload to our site or via FTP.

If we need to add code we can do this – we just need some guidance where! ??

Thanks,

Laurence