How do I change incoming truncated email messages

Please provide more information. What is popping up? Are you using an addon? Please share OS, and version of Thunderbird. A screenshot of what you are seeing may prove helpful. Thank you

Popping Up on the email body is a box labeled "Truncated" Truncated!This message exceeded the Maximum Message Size set in Account Settings, so we have only downloaded the first few lines from the mail server. Next it asks to click on the link to download the rest of the message.

Download the rest of the message.

Thunderbird version is 115.18.0 The server is AT&T

How would I correct this in the settings?

Popping Up on the email body is a box labeled "Truncated" Truncated!This message exceeded the Maximum Message Size set in Account Settings, so we have only downloaded the first few lines from the mail server. Next it asks to click on the link to download the rest of the message.

Download the rest of the message.

Thunderbird version is 115.18.0 The server is AT&T

How would I correct this in the settings?

It's in Account Settings , Synchronization & Storage, Disk Space  : you can uncheck "Don't download message larger than <N> kb or change the <N> to a greater value.

Is this one message or all messages? If just one, it might be a malformed message that can be removed by logging on to the server to remove it.