"I have downloaded a big texture already" - ok.
"but did not get corresponding DEM" - Why didn't you get it, and why did you should to get it? RWT didn't download it, or didn't you manually download it?
"Now, I only wanted to download only DEM and wanted to reuse downloaded texture which is existing somewhere in cache." - RWT tries to reuse what you already have in the cache. So if there are texture tiles in the cache but no elevation data, RWT will only download the elevations. You don't need to do anything for this. Just click Start.

I have downloaded a big texture already but did not get corresponding DEM...

Now , I only wanted to download only DEM  and wanted to reuse downloaded texture which is existing somewhere in cache..

Can I do the same?  if yes how?

To share the cache between projects, open RWT settings, select Cache Folder - Custom cache folder, and specify your folder.

Actually , I have downloaded a big texture but unfortunately elevation data was missed. Now, I wanted to re-use downloaded texture tiles and want to download elevation data of the same area.

pls guide.

To share the cache between projects, open RWT settings, select Cache Folder - Custom cache folder, and specify your folder.

Out of the box, RWT cannot use HGT files.
But you can force it this way:
https://forum.infinity-code.com/viewtopic.php?id=1538

Downloaded the textures and wanted to re-use. What shall I put it in custom settings ?

project is in e:\unity\folder\RWT_cache\

How to download DEM in hgt format ?

Simply placing the DEM in RWT_cache folder will help ?

regards,
Jai