You cannot recharacterize a Roth IRA contribution to be a SEP IRA contribution (although you could recharacterize it to be a regular traditional IRA contribution to the SEP IRA account, if your SEP IRA custodian permits regular, personal contributions to your SEP IRA account; of course it would be a nondeductible contribution). Roth IRA contributions are personal contributions while SEP IRA contributions are employer contributions. (A self-employed person is both employer and employee for the purpose of making retirement contributions.)
As I mentioned, you can recharacterize the Roth IRA contribution to be a nondeductible regular traditional IRA contribution. You could also request a return of the Roth IRA if you don't want to make a nondeductible traditional IRA contribution, but any earnings attributable to the returned contribution would be taxable and potentially subject to an early-distribution penalty.
I already successfully completed the recharcterization. I am self-employed.
As I said, a Roth IRA contribution that is recharacterized becomes a regular traditional IRA contribution, even if recharacterized to a SEP IRA account. It does not become a SEP IRA contribution.
Ignoring whether or not such a recharacterization is permissible, you would report the amount originally contributed to the Roth IRA as a SEP IRA contribution instead, and not report any Roth IRA contribution. Under the circumstances, TurboTax will not prompt you for an explanation statement regarding this, so you will need to prepare an explanation separately and mail it with your tax return since the Form 5498 from the SEP IRA custodian will not show any SEP IRA contribution to corroborate your self-employment contribution deduction. The Form 5498 will only show an recharacterization contribution.
I am in the same situation.. self-employed and made a roth IRA contribution last year that needs to be re-characterized. I started by recharacterizing to a Traditional IRA but just am now learning about SEP IRA's and that seems like a much better option. How did you do it? (I am using H&R Block for tax filing so if you have any insights on that too, it would be much appreciated!)