update 2 fields using linq foreach

Well to start with, assuming this is List<T>.ForEach, this isn't using LINQ. But yes, you can create a lambda expression using a statement body:

users.ForEach(x => {
    x.CreateTime = DateTime.Now.AddMonths(-1);
    x.LastUpdateTime = DateTime.Now;
});

However, you may also want to use one consistent time for all the updates:

DateTime updateTime = DateTime.Now;
DateTime createTime = updateTime.AddMonths(-1);

users.ForEach(x => {
    x.CreateTime = createTime;
    x.LastUpdateTime = updateTime;
});

It's not really clear why you want to achieve it this way though. I would suggest using a foreach loop instead:

DateTime updateTime = DateTime.Now;
DateTime createTime = updateTime.AddMonths(-1);

foreach (var user in users)
{
    user.CreateTime = createTime;
    user.LastUpdateTime = updateTime;
}

It's actually not linq, but you can try

users.ForEach(x => { x.CreateTime = DateTime.Now.AddMonths(-1);
                     x.LastUpdateTime = DateTime.Now; });