In fdsymbol.sty there are no slot numbers and one has to count manually.
One finds
\fdsy@setslot{0}
\fdsy@DeclareSymbol{\rightarrow}{arrows}{\mathrel}
which means \rightarrow is at slot 0 of the arrows symbol font. Looking for your desired symbol and counting yields that
\leftrightarrow is at slot 16
\updownarrow is at slot 17
\neswarrow is at slot 18
\nwsearrow is at slot 19
We also find
\DeclareSymbolFont{arrows}{U}{FdSymbolC}{m}{n}
Thus we need also to copy the font definitions for this font and undefine \updownarrow.
You may want to change the scaling value.
\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}
\makeatletter
\def\fdsy@scale{1}
\newcommand\fdsy@mweight@normal{Book}
\newcommand\fdsy@mweight@small{Book}
\newcommand\fdsy@bweight@normal{Medium}
\newcommand\fdsy@bweight@small{Medium}
\DeclareFontFamily{U}{FdSymbolC}{}
\DeclareFontShape{U}{FdSymbolC}{m}{n}{
<-7.1> s * [\fdsy@scale] FdSymbolC-\fdsy@mweight@small
<7.1-> s * [\fdsy@scale] FdSymbolC-\fdsy@mweight@normal
}{}
\DeclareFontShape{U}{FdSymbolC}{b}{n}{
<-7.1> s * [\fdsy@scale] FdSymbolC-\fdsy@bweight@small
<7.1-> s * [\fdsy@scale] FdSymbolC-\fdsy@bweight@normal
}{}
\makeatother
\DeclareSymbolFont{fdarrows}{U}{FdSymbolC}{m}{n}
\SetSymbolFont{fdarrows}{bold}{U}{FdSymbolC}{m}{n}
\DeclareMathSymbol{\leftrightarrow}{\mathrel}{fdarrows}{16}
\let\updownarrow\relax
\DeclareMathSymbol{\updownarrow}{\mathrel}{fdarrows}{17}
\DeclareMathSymbol{\neswarrow}{\mathrel}{fdarrows}{18}
\DeclareMathSymbol{\nwsearrow}{\mathrel}{fdarrows}{19}
\begin{document}
$\leftrightarrow\updownarrow\neswarrow\nwsearrow$
\end{document}

fdsymbol.sty. The sub-font containing the symbols you want isarrows, You will need to copy the lines that are equivalent to the ones shown in the cited question, but for thearrowssub-font and the particular symbols. – barbara beeton Jul 13 '19 at 17:26texdoc fdsymbolat a command prompt. If it's not there, you should be able to add it using the method for your installation; see the links here for a method appropriate for your system: Installing, updating, restoring, other system operations – barbara beeton Jul 13 '19 at 17:58