You can modify item properties, except ID like this.

    public override void ItemAdding(SPItemEventProperties properties)
    {
        string mycol_internal = "";
    
        using (SPWeb web = properties.OpenWeb()) {
            mycol_internal = web.Lists[properties.ListId].Fields["My Column"].InternalName;
        }
    
        properties.AfterProperties[mycol_internal] = "your new value";
    
        //Update title with another column value etc
        properties.AfterProperties["Title"] = properties.AfterProperties[mycol_internal].ToString();
    }
[Source][1]

OR, use ItemAdded where `properties.ListItem` is available.



  [1]: https://social.msdn.microsoft.com/Forums/sharepoint/en-US/a27338c1-4a11-43f6-a270-8c6957ee64e3/problem-about-current-item-in-itemadding-reciever