.
In[1]:= A = (Log[s + t] + s2 + t s + u);A /. Log[s + t] -> Log[a + b] Out[2]= s2 + s t + u + Log[a + b]
Or
In[2]:= (Log[s + t] /. {s -> a, t -> b}) + s2 + t s + u Out[2]= s2 + s t + u + Log[a + b]
