If the proportion has a (a, b) prior and one observes Y from a distribution with parameters n and p, then if one observes Y = y, then the posterior density of p is (a + y, b + n - y).
Recall that the mean of a Beta(a, b) random variable following is a / (a + b). Show that the posterior mean of p | Y = y ~ Beta(a + y, b + n - y) is a weighted average of the prior mean of p ~ Beta(a, b) and the sample mean p̂ = y/n. Find the two weights and explain their implication for the posterior being a combination of prior and data.