Hi,
when you update your IPS to 4.5.x for first time you need to setup marketplace in ACP from there you can link your existing apps/themes/plugins to ips marketplace then simply clicking the update button will work.
if you have missed that and you didnt link your theme to ips marketplace, then there is nothing else to do other than installing your theme fresh from marketplace and copy paste your settings, customizations...etc from the old one to the new one