You can use a package like CMS Import to do it all via the UI, or programmatically you can do something like this:

IContentService cs = ApplicationContext.Services.ContentService;

IEnumerable<IContent> affectedContent = cs.GetContentOfContentType(1234);

foreach (IContent content in affectedContent)
{
    foreach (PropertyType pt in content.PropertyTypes)
    {
        switch (pt.Alias)
        {
            case "propertyAlias1":
                foreach (Property p in content.Properties.Where(x => x.PropertyType.Id == pt.Id))
                {
                    content.SetValue(p.Alias, "New value");
                }

                break;
            case "propertyAlias2":
                foreach (Property p in content.Properties.Where(x => x.PropertyType.Id == pt.Id))
                {
                    content.SetValue(p.Alias, "New value");
                }

                break;
        }
    }

    if (content.Published)
    {
        cs.SaveAndPublishWithStatus(content, 0, false);
    }
    else
    {
        cs.Save(content, 0, false);
    }
}

Is this really the most efficient way?

I've been giving this quite a bit of thought recently as I have a large load balanced site that imports around 1000 nodes at a time from a SalesForce instance and this approach is slow and costly but I can see no other way around it at the moment. It massively impacts the site because each publish action drops the cache and triggers all of the required event handlers (this is required but would rather it were performed in a single hit)!

What I would like to do is perform the updates and save changes then pass all Id's to a method that would bulk publish them but on last check this was not possible?

is working on a reply...