You can start the "update" whether in InitSetting screen
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Or in KK-Manager
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Now there is first the P2P setting to confirm, then wait for the Update List (may take several minutes)
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The P2P transfer has the advantage that the download itself may be significantly faster (but still may take several hours).
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In this list you have to activate which pack you want to update (for example let the Studio Items unactivated, if you don´t have the Optional Studio Modpack Studio).
Now click on a Modpack Name and get listed below which new mods you get with this update (green text), and especially which mods get deleted (red text)
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Red texted mods to delete pls note: these are outdated mods, maybe you find the same as green text in the list - so you get the newer version, the old version get deleted.
But be careful about the only red texted - I always check them through and eventually make a backup before starting the update to avoid loosing them completely.
This function is the cause you should store self downloaded mods in mods/MyMods to prevent them from deleting while not listed in the Official Sideloader Mods Collection.
Then start the update - you can leave it alone (can last several hours), get a message if download/automatic install is completed.