I don't think you need any sort of upgrade or flashing before you can download or get a compatible for wallet for windows phone. Try this wallet coin space
Merit me if you find it helpful.
Well this is not what i'd call a meritable post at all. You "don't need" any kind of other OS, but think about it like as being like Windows XP. Of course there are apps for it, but it does not mean that it is supported and should be used really. All of the Windows Phone apps that i have tried, have been really buggy and i would not trust them (or the OS for that matter) with any kind of funds.
Also the Coinspace which you suggested, is a new application and probably have not got their source code audited.
So Copay and Bitpay are the best bets for wallets, if OP wishes to continue on the OS which he has currently. Copay has 2 Out of 5 stars, while Bitpay 3 out of 5, so it is obviously sure that this is a VERY bad idea, at least if they are used you should verify the backup of the seed.