First select your record you want to update:
var tabmax = context.Tabmaxes.Where(t => t.tabelle == "AAGFAKT" && t.mandant == null && t.krit == aAGAUFART).FirstOrDefault();
Then change it:
tabmax.tabmax1 += 1;
Then update it:
context.Update(tabmax);
context.SaveChanges();
