Hi and Welcome to the forum.
You cannot see your top up history in your Myo2 App or via the link to the myo2 at the top of the forum page. If you purchased your voucher from the shop by way of debit or credit card then you should be able to check your bank statement to see what date you bought it which should give you an idea of approximately when you used it to top up.
If you call Customer Services on 4445 they will be able to check your last top up and cross reference that with your anniversary date and amend it if required.
Or use Live Chat
http://www.o2.co.uk/contactusIf you topped up late then your anniversary date does change to your latest top up date.