So you want your non main save slot to be your main one? Well you have come to the right place!
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1. Open up the game, and launch a mission with the save that you want to be your main one, so the game creates an auto save point
2. After returning to the space rig, select the 𝐋𝐨𝐚𝐝 button for the 𝐌𝐚𝐢𝐧 𝐒𝐚𝐯𝐞 slot
3. Go back into options and then save menu. Your 𝐌𝐚𝐢𝐧 𝐒𝐚𝐯𝐞 should now also be the 𝐀𝐜𝐭𝐢𝐯𝐞 𝐒𝐚𝐯𝐞. Now click the 𝐑𝐞𝐬𝐭𝐨𝐫𝐞 𝐒𝐚𝐯𝐞 button.
4. In the 𝐑𝐞𝐬𝐭𝐨𝐫𝐞 𝐒𝐚𝐯𝐞 menu, find the most recent save, and click it. (it may not always be on the top)
5. The game will ask you if you would like to overwrite your current save, click 𝐘𝐞𝐬.
6. After a short loading screen, your alternate save slot should now be the same as your main one. Enjoy!