Is it for the tax due or did the IRS change your return and say you owe more?

per IRS

What you need to do
Pay the balance due within 10 days using the envelope provided.
If you’re already working with us to resolve the balance, no action is required.
If you’ve already paid the balance, call the telephone number listed on the letter.
If you can’t pay the balance due, call the telephone number listed on the letter.

https://www.irs.gov/individuals/understanding-your-lt39-notice