Wikipedia defines a coup in the following way:
A coup [...] is a seizure and removal of a government and its powers. Typically, it is an illegal seizure of power by a political faction, rebel group, military, or a dictator.
Their definition of regime change is similar:
Regime change is the forcible or coerced replacement of one government regime with another. Regime change may replace all or part of the state's most critical leadership system, administrative apparatus, or bureaucracy.
A coup or regime change of the Russian government would likely have a significant impact on the 2022 military conflict in Ukraine.