{
	"@odata.context": "http://portal.xpertdoc.com/odata4/v6/$metadata#ContentMetadataForms/$entity",
	"ContentMetadataFormId": "00000000-0000-0000-0000-000000000000",
	"ContentLibraryId": "00000000-0000-0000-0000-000000000000",
	"Name": "Name",
	"Description": null,
	"Metadata": "<Form><Group DisplayMode='Minimal' Expanded='true' Label='Default' Name='Default'><Fields/></Group></Form>",
	"CreatedByUserProfileId": "00000000-0000-0000-0000-000000000000",
	"CreatedDate": "2016-01-01T00:00:00.0000000Z",
	"ModifiedByUserProfileId": "00000000-0000-0000-0000-000000000000",
	"ModifiedDate": "2016-01-01T00:00:00.0000000Z",
	"Timestamp": "AAAAAAAEqhM="
}

var contentMetadataForm =  portalContext.ContentMetadataForms.Where(clf => clf.contentLibraryFolderId == Guid.Parse("00000000-0000-0000-0000-000000000000")).FirstOrDefault();

contentMetadataForm.Name = "New Name";
contentMetadataForm.ValueToChange = value;

portalContext.UpdateObject(contentMetadataForm);
portalContext.SaveChanges();