Editing a 45 minute vocal file

I purchased a cassette player that attaches to my PC with a USB cable. Play the old cassette and it creates an audio file using Audacity.
Unfortunately the player is also powered by USB and on longer tapes it starts to slow down causing the playback to of course sound funny. I can speed up the playback making the voice sound normal, but how do I save it in a new file?
I open the file in Audacity. The first 6 minutes the voice sounds fine then it starts to slow down and sound funny. I highlighted from that point till the end and changed the playback speed to a little faster where it sounds pretty normal. I export to a new file, but when I play that new file back the speed up isn’t there.
How do I make the speed change after 6 minutes in and save it in a new file?

PS: I’m using the new 2.0

You can get production ambiguities like that if you try to save the corrections with the same file name as the original work. Pick a different name. Koz