PUSHD and POPD should help in your case.
@echo off
:: Create a temporary drive letter mapped to your UNC root location
:: and effectively CD to that location
pushd \\server\soft
:: Do your work
WP15\setup.exe
robocopy.exe "WP15\Custom" /copyall "C:\Program Files (x86)\WP\Custom Templates"
Regedit.exe /s WPX5\Custom\Migrate.reg
:: Remove the temporary drive letter and return to your original location
popd
Type PUSHD /?
from the command line for more information.