Turn the gas off at the meter, unplug the bayonet fitting, turn the gas on, check for leaks with washing-up liquid (even small amounts of leaking gas will form bubbles), and if in doubt turn the gas back off and phone a gas man...
Electrickery's easier, as it tends not to leak out of stuff. If the oven's hard-wired, turn the power off, disconnect the cable from the isolator switch, maybe fit a grommet or something to fill the hole if live stuff would otherwise be exposed, job done. No need for any kind of certification to replace an existing fitting, and disconnecting an oven surely qualifies.
But sure, if any of this seems scary, GAMI.