If you don’t do it right, it just wasn’t done.
A tenant with an option to extend a Lease must strictly comply with the terms of the Lease with respect to how to exercise that option right. If the Lease is silent on how, the tenant can merely call the landlord and say that the tenant is exercising the right to extend.
If, however, the Lease calls for written notice, or notice by carrier pigeon, that’s how the tenant must give the notice to the landlord.
If the tenant doesn’t, the extension right is lost forever — even (unless there are very exceptional circumstances) if the tenant stays in the space and continues to pay rent based on the extension rent rate for months, or years, after the end of the first Lease term.