Try to remove them with the engine hot.
I prefer changing plugs with the engine cold. If room temperature plugs are installed and torqued to spec on a hot engine the aluminum head will shrink as the engine cools and the plug heats up and expands, which will combine to increase the head's grip on the plug, especially with taper seat plugs.
If you can get them out, let the engine cool to near room temperature. Then install the new plugs and torque them to the minimum spec using a torquewrench and the published spec range. It may be in your owner's manual. The spec is in my owner's manual - 10-20 NM (7-15 lb-ft) for the M103 taper seat plugs. Look in the index under "spark plugs".
For stuck plugs it's best to use a T-handle and apply equal force to each side, so you don't place a bending moment on the plug, which can break off the insulator. If you use a conventional breaker bar, support the pivot point with your free hand to mininmize the bending load that you place on the plug.
Also, use a large 1/2" drive T-handle or breaker bar and put a shock load on it - like an impact wrench. A shock load is more likely to break the plug loose than a steadily increasing pull.
Duke
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