How to set value of metadata column programmatically

A Managed Metadata column is a new column type that can be added to lists, libraries, or content types to enable site users to select values from a specific term set of managed terms and apply these values to their content. You can create and configure a Managed Metadata column to map to an existing term set or term, or you can create a new term set specifically for a Managed Metadata column.

using(SPSite site = new SPSite("http://myspsite")) {
    using(SPWeb web = site.OpenWeb()) {
        //Name of list - testlist
        SPList list = web.Lists["testlist"];
        //Get item ID - 2
        SPListItem listItem = web.Lists["testlist"].GetItemById(2);
        //Get column nameof list - testlist
        Guid Promotypeid = listItem.Fields["testlist"].Id;
        TaxonomyField taxfield = listItem.Fields[Promotypeid] as TaxonomyField;
        TaxonomySession session = new TaxonomySession(site);
        TermStore mytermstore = session.DefaultKeywordsTermStore;
        TermSet termSet = mytermstore.GetTermSet(taxfield.TermSetId);
        Guid termGuid = Guid.Empty;
        //update item string value from Term set (value must exist in term set)
        string ChangeValue = "Pulsar 250";
        //Find the term from tremset and get GUID
        foreach(Term term in termSet.Terms) {
            if (term.Name.Equals(ChangeValue, StringComparison.OrdinalIgnoreCase)) {
                termGuid = term.Id;
                break;
            }
        }
        if (termGuid != Guid.Empty) {
            Term customTerm = termSet.GetTerm(termGuid);
            string taxFieldInternalname = listItem.Fields[taxfield.TextField].InternalName;
            listItem[taxFieldInternalname] = customTerm.Name + "|" + customTerm.Id.ToString();
            taxfield.SetFieldValue(listItem, customTerm);
            web.AllowUnsafeUpdates = true;
            listItem.Update();
            web.AllowUnsafeUpdates = false;
        }
    }
}