Need help with S4 Studio / Blender (want to remove relief)

That is the bump map (or normal map) causing that overlay effect.

You don't need blender to open the file up, it's a texture.

If you go to the warehouse tab, look for the DST files on the left, there should be 6

• 3 are for the diffuse image (one for your image, one for the Normal Map, one for the Specular Map)
  
• 3 are for the glass (one for the glass texture, one for the Normal Map and one is for the Specular Map)
  

• Diffuse Map is the "texture" you're replacing
  
• Normal map gives objects that are flat a 3D appearance
  
• Specular map gives items a shine or matte appearance
  

The easiest way to get rid of that overlayed "bump" is to replace that DST image with a blank one.

(You can download them from S4S, at the top of this link)
https://sims4studio.com/thread/1079/get-eas-bump-specular-maps

If you click one of the DST files in the Warehouse tab of S4S you will get a preview of it, so you can tell what is what

• Download the Blank Normal Map
  
• Make sure you have the correct DST file highlighted on the left (in this example, it would be the 2nd one)
  
• At the bottom right, click the IMPORT BUTTON
  
• Find the location you saved the Blank Normal Map on your PC to Import it like you do with textures
  
• Save Package