kuruman

Let's take it from the top.

KE = (1/2)mv²; PE = mgh

The total mechanical energy is constant, KE + PE = Constant

Therefore when the pendulum goes higher, two things happen PE increases because h increases and KE decreases to keep the sum KE + PE constant.

But if KE decreases, v must also decrease.

Now in terms of angular velocity ω, v = ω r.

Here, r is the length of the pendulum. This stays constant as the pendulum swings back and forth.

So ...

Looking at v = ω r, if v decreases and r stays the same, what happens to ω?