Because the Attribute is read-only, you will not be able to change its value. Instead, consider creating a 'new' Asset, set its Attributes, and then save it.
Review the example below and attempt to utilize the idea within your project:
var TestId = Oid.FromToken("Test:26017", _context.MetaModel);
var TestAsset = _context.MetaModel.GetAssetType("Test");
var newTestAsset = _context.Services.New(TestAsset, TestId);
var TestStatusAttr = newTestAsset.GetAttributeDefinition("Status.Name");
newTestAsset.SetAttributeValue(TestStatusAttr, "Failed");
_context.Services.Save(newTestAsset);