Upgrade v4.3.8 to v4.4

I have tried twice to upgrade from V4.3.8 to v4.4 and get a message environment error, I have searched everywhere but unable to find any reference to this.
We have a few applications using v4.3.8 which I am assuming will, at some point need to be updated.
A clean install of v4.4 on the same development system works fine, so not PHP version I don't think, unless there is another module needed.
Thanks for an help you can offer.
Alan

See changed files https://codeigniter4.github.io/userguide...ll-changes

Thank you I did look at the update but must have missed this option, I will give it a go, sorry if this was an obvious fix.
Alan