Then follow the instructions below!

ioquake3 is an updated engine for Quake III to run on, making it possible to display the upscaled textures (and run other mods) without issue. If you try to use these textures with just the base engine you will get errors so ioquake3 is a must. There are other engines out there like Quake3e but I get errors trying to use those so I definitely recommend using ioquake3 over anything else.

Step 1
Download ioquake3: https://ioquake3.org/get-it/
Download newer PK3 files (Scroll down and press "I Agree". If you don't do this step it will bug out when trying to open Team Arena): https://ioquake3.org/extras/patch-data/

Step 2
Unzip the archives using WinRAR or a similar program and copy and paste the files inside the unzipped folders into your base Quake 3 Steam installation folder (usually C:\Program Files (x86)\Steam\steamapps\common\Quake 3 Arena). For installing the newer PK3 files make sure you click yes to REPLACE the already existing files.

Step 3
Now, rename the quake3.exe file to “quake3 (rename to just quake3 to launch with Steam)” and rename the “ioquake3.x86_64” file to “quake3”. This will make it so when you press the Play button in Steam it actually launches ioquake3 and not the original.

Download these mods:

Unofficial Patch: https://www.moddb.com/mods/the-unofficial-patch/downloads
HD Menu/HUD: https://www.moddb.com/mods/high-quality-quake/downloads
If you also have Team Arena installed get the HD Menu for that awell: https://www.moddb.com/mods/high-quality-quake/addons/hqq-high-quality-quake-team-arena-test

Copy and Paste the files into your base Quake 3 Steam installation folder.

Step 1
Download the Player, World, and Object textures from this link:
https://www.moddb.com/mods/quake-3-neural-upscale/addons

Put the files into the “baseq3” folder located in your Quake 3 Steam installation folder.

Step 2
Click the Settings cogwheel to the right of the Play button on the game's Steam Library page --> Properties --> General --> Launch Options --> copy and paste this into the text box:

+set com_zoneMegs 128 +set com_hunkMegs 192

If you don't do this step the game will crash when trying to load the map Q3DM4.

High Quality Sound Replacement Pack: https://www.moddb.com/mods/high-quality-quake/addons/srp
(Test this one out to see if you like it. It gives a lot of weapons like the machine gun and shotgun more "punch" which sounds better but then for the Lightning Gun it removes the lightning/thunder sound that is supposed to play when it starts firing so it's just up to you and what you like better).

Old Quake Live Announcer Voice for Quake 3: https://www.moddb.com/mods/quake-live-announcers-for-quake-3/downloads/quake-live-announcer-old-voice

I hope this guide was helpful and that you enjoy the updated look! :D

I also recommend checking out my other guide on how to fix the Resolution, FOV and FPS (no modding required so it works in the base game aswell): https://steamcommunity.com/sharedfiles/filedetails/?id=2959296140