The form of the will is either simple written form (personal wills), or an act certified by a notary public (notarized wills). The last are kept in the archive of the notary public and eventually, registered in national list of notarized wills.
Wills can be withdrawn or substituted with other wills by the same testator, made later (usually where the testator has changed their mind about the chosen beneficiaries of estate). In such situations only the latest signed will takes effect.
Usually a will may be invalidated if it affects the shares granted by the Law (depending of the jurisdiction) or where made not in the recognized form by the Law, or where prepared via fraud or in situation of mental incapability/incompetency.
Wills may be executed either by the beneficiaries, or by specifically chosen individuals.