They should remind you somehow whether it be by text/email or call. I can't remember as normally I check the date and upgrade early before the date(think it's something like 2/3 weeks before you can upgrade).
If your unsure just ring them and say when is the earliest I can upgrade, then they'll give you an exact date, ring on that date to upgrade, job done.
Also worth noting, if your happy with your current phone you could instead of accepting a new handset, ask them if you can lower the monthly bill but keep the same mins/texts/data. I did that with Three once, ok it was only something like £3-4 less per month but it adds up abit over 2 years.
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If your unsure just ring them and say when is the earliest I can upgrade, then they'll give you an exact date, ring on that date to upgrade, job done.
Also worth noting, if your happy with your current phone you could instead of accepting a new handset, ask them if you can lower the monthly bill but keep the same mins/texts/data. I did that with Three once, ok it was only something like £3-4 less per month but it adds up abit over 2 years.