The custom skin files are located in an FTP account. To connect to FTP, you should download a software like Filezilla or similar.
Using the credentials we have provided, for example:
Once logged in, you can make the modifications you want locally and then you have to upload them back to the FTP.
Note. making a copy of the original custom skin files can be useful for backup purposes, in case changes are made. However, it is always better to have an expert developer handle any modifications.