Can I change the distribution description code for Box 7 on a 1099-R? The early distribution was due to my disability and the code for Box 7 should be "3".

No, you are not permitted to change the code.  If you think that the code is incorrect, contact the payer for correction.

Otherwise, go to Other Tax Situations -> Additional Tax Payments -> Extra tax on early retirement distributions and enter the amount in the Permanent Disability box.

You'll also need to provide the IRS with documentation to substantiate that your disability makes you:

unable to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or to be of long-continued and indefinite duration.

That documentation would be a statement from your physician, mailed with your tax return.