You have to tell the bank at the time you made the contribution, that it is designated as a prior year contribution. This may be as simple as a drop-down menu in online banking, or you might have to mail a check with a special form, depending on the bank. You can't fix it retroactively.
First you need to file an amended return for 2017 that removes the contributions, adds them back to your taxable income, and pay the tax.
Then, since this contribution is official a 2018 contribution, it is applied to your 2018 limit. If you have ended up contributing too much for 2018, you have to make a "withdrawal of excess contributions". This also requires a special form or procedure, it is not a normal withdrawal. You will report the withdrawal of excess contributions on your 2018 tax return and they will be included in your taxable income.
