Plenty of other people have made great videos on how to do this, but it is still a question I get a lot, so hopefully this helps : )
DISCLAIMER: I've heard of some games / communities treating this as exploiting. I haven't played any of these games and I'm not sure how they detect or enforce it, but just be aware. I do it for the better aesthetics (IMO) and any developers who rely on textures to balance their games should consider alternate methods anyway (also IMO).
Step by step (text):
Step 1. Make sure you can view hidden files and folders. In Windows Explorer, Click the Options icon. Then, go to the view tab. Under Hidden files and folders, click "Show Hidden files and folders". Hit Apply and OK.
Step 2. In Windows Explorer, go to This PC, then your Local Disk (See). Users. Your user name. AppData. Local. Roblox. Versions. You may have multiple version folders here with random numbers. Just go to the one with the most recent Date Modified. Platform Content. PC. Textures. Shift-click and select all of the folders, but not the DDS files. Either delete this folders or move them to a different spot.
Step 3. Drag the main Versions folder to your Quick Access on the left.
Step 4. Unfortunately, your textures will be re-installed every single time Roblox updates. I know this is annoying. Instead of manually having to find your AppData Roblox folder each time, I go back to the saved shortcut that we did in Step 3, so it's very easy to re-delete them again.
Optional Step 5. If you want your textures back, it's as easy as reinstalling Roblox. Or, if you moved them to another folder in Step 2, you can just move them back.
Games in this video:
Mech Warzone by Overheat
https://www.roblox.com/games/3032856162
Unit: Classified by ROLVe
https://www.roblox.com/games/4292776423
Impulse: Remastered by Rhyles
https://www.roblox.com/games/3204503125
0:00 Intro
0:30 What's this about?
1:03 Steps
2:52 Random gameplay - Impulse: Remastered
Recorded with Windows 10 game bar, edited with HitFilm Express.
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I used the Roblox FPS unlocker to get smoother gameplay (highly recommended if your computer is OK): https://github.com/axstin/rbxfpsunlocker
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PC Specs
Processor: i7-6700K
RAM: 16 GB
Graphics: Nvidia GTX 1070