For such recent deaths, the information entered onto computer by the local office is what GRO also see so the information should always match what is on the register entry. But there are scenarios I can imagine e.g. a problem with the registration computer system, RON, on that day (which isn't that uncommon) meaning the entry was done on paper and later transferred into the system with an error, but changing 1935 to 1917 isn't an obvious mistype.
It might be a coincidence that she was 79, but the error makes her 97 which does suggest maybe a transposition of those numbers at some point perhaps when her year of birth was being worked out - the age is not recorded on the entry, it is now done as the date/year of birth.
The GRO index will only be updated if it doesn't match the register entry, not just because you know the entry itself is wrong, so the first thing to do is to get a certificate to see what that says so you know where the error may have occurred.
If the registration itself needs correcting, you will have to make an application to the Supt. Registrar who holds the register for a formal correction. As the year of birth is more than one year out, I think it will be a "complex clerical error" which requires the Registrar General to give authority for the change.