The rule is simple: a shorter pendulum swings faster. Raising the bob makes the clock gain. Lowering it makes the clock lose.
Most pendulums have a rating nut under the bob. Turning it so the bob moves up speeds the clock, and turning it so the bob moves down slows it. Many clocks are marked 'S/F' (slow/fast) or, on French clocks, 'A/R' (avance/retard).
Always stop the pendulum before touching the nut, and support the bob so the suspension spring does not take any strain.
Make small changes: a single turn can change the rate a lot. Adjust, let the clock run for a full day, compare it with an accurate reference, then adjust again.
Some clocks, especially French and carriage movements, are regulated from the dial through a small square arbor marked A/R. Turn it gently with the correct key.
Temperature changes the rate of uncompensated pendulums, so small seasonal variation is normal.