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.
