This does not seem to be DirectAdmin related question; I've moved it to Off-Topic Discussion.
You should be able to get information on using FTP or perhaps the DirectAdmin Files facility to download your files to your computer.
I'm not sure who you should ask how to use them on your computer; after all it's yours, and you should know how to use your own tools.
Once you've made any changes on your files you can upload them to your account on the webhosting server using either FTP or the Files facility. Again, you can get support on that from your hosting company.
Jeff