How do you brake loose a lawn mower blade?

It depends on the make of the mower. Some use a central lock nut similar to that used on an F1 wheel. The nut is undone by turn in the opposite direction to the rotation of the blade. To prevent blade turning use a wood chock to stop it as it apply force to the nut. If the nuts jammed use a loosening agent (oil or WD40) first and allow to soak in

the other option is 2 to 4 bolts or screws holding the bald to a shaft. these need to be looked for and cleaned as dirty and grass will have blocked then. Remove using a wd40 first to help loosen rusty threads