If you are in the diaspora or out of the country and you want to register or update your Safaricom Simcard details online then (currently) the only you have is to send Safaricom a message via Twitter @Safaricom_Care and they will help you with the registration process.
Note: You will be required to provide your phone number, ID/passport number, and attach passport with (visa and biodata, exit stamp from Kenya, and entry stamp to the country you are). You will also be required to provide a contact number that can be used to reach you.
Update your SIM registration details here, online