My questions

(1) What is the best way to add new textures into DooM2 on Windows 10
(2) Same with new sounds.
(3) How do I change the name of the levels episodes?

I have DooM Builder and Slade.

What map format are you using?

1. http://doomwiki.org/wiki/Adding_custom_textures
2. Simply import the sounds into your wad via SLADE3. Name the lumps the same as the stock sound lumps that you wanted your sounds to replace. If your wad is for ZDoom and you wanted your sounds to be more than just replacements of the originals, you may also use SNDINFO to define new logical sound names and DECORATE to assign them to particular thing types.
3. If you want cross-compatibility, use DEHACKED to change automap map names + intermission story texts (and replace the CWILV** intermission graphics with your own, manually made ones), or if you're making a wad specifically for ZDoom, use MAPINFO (or possibly LANGUAGE). DEHACKED is best to be edited in an editor like WhackEd, while MAPINFO (and LANGUAGE) can be easily edited in text form. In either case, make an empty lump inside your wad named either DEHACKED or MAPINFO, and put/import the respective script into it.

Documentation/reference to all ZDoom-specific lumps can be found on the zdoom wiki: http://zdoom.org/wiki/Main_Page

3. I think episode names are stored in a graphic format inside the IWAD. But maybe I got you wrong.

So that's what he meant by "the name of the levels episodes" - the Doom 1 episode names (I thought he meant intermission texts). The Doom 1 episode names in the menu are indeed just graphics in the IWAD, replacing them would be the same like replacing any other graphic, by putting a new graphic lump into your wad with the same name as the original lump's name in the IWAD. Again, ZDoom has an option to change those names via MAPINFO, the new texts would get generated automatically and replace those original graphics.