How can I use a specific mirror server in Fedora?

There are multiple options, you can hard code it (/etc/yum.repos.d/fedora.repo) or using fastmirror.

In order to use fastmirror you must first activate it:

in /etc/dnf/dnf.conf add fastestmirror=true then using this command dnf config-manager --add-repo add the mirror you want to use, you can add multiple mirrors and the fastest one will be used.

Example:

sudo dnf config-manager --add-repo http://ftp.byfly.by/pub/fedoraproject.org/linux/releases/23/Everything/x86_64/os/

You must use the path /linux/releases/23/Everything/x86_64/os/ for fedora 23 releases and for fedora 23 updates /linux/updates/23/x86_64/ otherwise will not work.