Do pilots maintain the same cruising speed and altitudes during a flight?

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Do pilots maintain the same cruising speed and altitudes during a flight? or can they change it because of weather and other?

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Depends on the length of the flight - on long hauls its common for planes to “step-climb” to higher cruise altitudes as they get lighter.

Yeah changing altitude to get away from e.g. turbulence is pretty common.

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You can request deviation to your flight path to avoid weather. You can also deviate from your vertical path to resolve TCAS. And then, there is SLOP on the NATs to avoid wake turbulence.

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They in fact, do not.

The higher the cruising altitude, the less the environment impact, due to more efficiency. The main reasons why planes don’t just fly this high is:
-Load may be too high
-Aircraft is not capable to do so

This is why altitudes can change when the fuel is burned away. Now that they are lighter, they can climb higher for more efficiency. This is called a step-climb.

Like you also asked, cruising speeds and altitude can also change due to weather. Maybe headwind/turbulence on a different FL is less, for example.
I may be a little inaccurate in some areas. Please correct me if I am.

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Pilots do not always maintain the same cruising speed and altitude throughout a flight. Several key factors influence these parameters, requiring adjustments for efficiency, safety, and regulatory compliance.

  1. Winds

Wind conditions play a major role in cruise altitude and speed. Tailwinds can increase groundspeed and improve fuel efficiency, while headwinds may require adjustments to maintain optimal performance. Pilots often adjust altitudes to find the most favorable wind conditions.

  1. Fuel on Board

As fuel burns off, the aircraft becomes lighter, allowing it to climb to higher, more fuel-efficient altitudes. This process, known as a step climb, is common on long-haul flights to optimize performance.

  1. Aircraft Type

Each aircraft has a specific optimum cruise altitude and speed based on its aerodynamics and engine performance. Larger aircraft generally cruise higher, while smaller aircraft may have lower operational ceilings.

  1. Load and Weight

A fully loaded aircraft at takeoff may cruise at a lower altitude initially and gradually climb as it burns fuel. Load also affects the optimal cruising speed, as a heavier aircraft may require higher thrust to maintain altitude.

  1. Airspace Regulations

Pilots must comply with ATC instructions, assigned flight levels, and airspace restrictions. In busy corridors, they may be required to adjust altitude or speed to maintain safe separation.

Because of these factors, cruising speed and altitude are not fixed but dynamically adjusted throughout the flight for efficiency and safety.

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Like do airliners ever go above mach 0.86 at cruising altitudes

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I would think this is rare.

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You would almost never see an aircraft cruising faster than mach .86 due to:
-Decreased fuel efficiency
-High TAS
-High engine N1

High engine N1(rotation speed of the inner turbine) and TAS(true airspeed) can cause damage to the aircraft, while airlines usually won’t cruise faster than mach 0.85 for maximum fuel efficiency(decreasing costs)