Your saves are located at:

This is how your Folder should look like. May have more or less files depending on how many characters you created.

profile.sav stores character and inventory related data.
save_#.sav stores campaign, adventure mode and quest item related data.
.bak files are backup files and can be ignored for this purpose.

By default the saves are assigned to these slots:

• save_0.sav -> slot 1
  
• save_1.sav -> slot 2
  
• save_2.sav -> slot 3
  
• save_3.sav -> slot 4
  
• save_4.sav -> slot 5

If you have deleted characters before, it might look a bit different.
For example, if you had 5 characters and deleted slot 1 to 4, then you would end up with save_4.sav displaying in slot 1 as your only character.

But it will always be lowest save_# from left to right.

If you are unsure what save you have to replace it's best to backup all saves and either trial and error until you replace the right one or just replace all.

1. Backup your save_#.sav file by either just moving it to the Desktop or simply copy & paste it in the same folder.

2. Download one of the saves below and unzip it.

3. Rename the downloaded save, if necessary from save_1.sav to match your saves #.

4. Drag the downloaded save to your save location and replace your save.

5. Boot up the game, if you haven't already.
Hit "Start Game", and it should load whatever save you downloaded. If you somehow still load in in your world that means you probably replaced a wrong save, the # was wrong.
Also it's enough to be in the main menu, when replacing the saves. No need to close the game. The Game is checking the save file every time you click on "Start Game".

6. Whenever you done. Restore your main save.

Video version

Edit: Added Iron, Forged Iron, Galvanized Iron, Hardened Iron, Relic Dust, Luminate, Corrupted Luminate, Simulacrum and Moon Essence to Cass's store.

Cass Selling 10x Corrupted Shards & Upgrade Materials[drive.google.com]

Simulacrum Farm[drive.google.com]
Simply to your right around the corner.

Corrupted Luminate Farm & Scrap[drive.google.com]
Run to the first structure, drop down and kill the 2 Aberrations and pick up the ring for extra scrap.