At the user's home directory (e.g. C:\Users\\ on a Windows machine, or /Users/ on a Mac) there is a hidden directory, called .m2. Within this directory there is a settings.xml file. It may be that this file is not existing, than just create a new text file and save it as settings.xml
Withing the XML file, the following notation allows to manually specify a mirror: see attached image.
The reference of the xml elements is:
mirrorOf: The server ID of the repository being mirrored (e.g "central")
name: A name of the mirror (optional, just for documentation purpose)
url: The url of the mirror
id: A freely selectable id of the mirror entry
Please check the attached file for an example.