You can use a sql script(useable if nodes are on the same level, otherwise you have to replace double values because this script only replaces parentID's):

drop table #temp
set nocount on

declare @OldParentId int
declare @ParentId int
declare @Id int
declare @Path nvarchar(150)

select @OldParentId = 1360 -- Set to the existing parent node
select @ParentId = 1412 -- Set to the new parent node

-- First update the parentid column
update umbracoNode set parentid=@ParentId where parentId=@OldParentId

-- Next update all the path columns for the children.
SELECT id,[path] into #temp FROM [umbracoNode] where parentid=@ParentId

while exists(select Id from #temp)
begin
    select @Id=Id,@Path=[Path] from #temp

    select @Path = REPLACE(@Path,@OldParentId,@ParentId)
    update umbracoNode set path=@Path where id=@Id

    print cast(@Id as varchar)+': ' +@Path

    delete from #temp where Id=@Id
end

Otherwise, you can use the umbracoAPI to replace nodes:

Document doc = new Document(docID);
doc.Move(newParentID);

doc.Save();

// only publish document if document has been published allready
if (doc.Published)
{
    doc.Publish(new User(0));                
    umbraco.cms.businesslogic.cache.Cache.ClearCacheItem("ApplicationTreeCache");
    umbraco.library.UpdateDocumentCache(doc.Id);
}