How do I change the bitrate field of an mp3 file?
Sorry but if the file has been encoded at the wrong bitrate, then they'll need to be re-encoded i.e. you'll need to rip them from a CD again. The bitrate refers to the amount of data that is used to store a second of audio. There is no way to increase this, in any meaningful way, once it's been created.

However if the file are encoded with Variable Bit Rate (VBR) encoding there does seem to be a common problem where the bitrate is misreported. If I'm reading things correctly it's because software like iTunes just looks at the bitrate of the first chunk of the file (which might just be silence or something simply resulting in a low bitrate encoding) and report that as the bitrate even though subsequent chunks might have completely different (higher) rates. There does appear to be a tool to fix this called vbrfix. However there doesn't seem to be an easy to get this tool for Macs.

If you are relatively confident with using the Terminal you could try the following:


Download http://http.debian.net/debian/pool/main/v/vbrfix/vbrfix_0.24+dfsg.orig.tar.xz
Decompress the file with something like The Unarchiver (macOS native Årchive Utility won't know what to make of it)
Open a Terminal window and cd to the path of the unarchived files
Run ./configure and then make to compile the vbrfixc binary
Assuming the binary compiles successfully cd into the "vbrfixc" folder and you should find the vbrfixc binary.
Run it with ./vbrfixc -log input.mp3 output.mp3.


If the file is a VBR MP3 file this will attempt to fix it by placing a blank chunk at the beginning of the file with a bitrate that is more representative of the entire file.