How can I remedy a Roth IRA contribution which isn't allowed?

Yes, you can withdraw (undo) the Roth contribution including earnings, and instead contribute to trad -- but only the standard amount, $6k, without any added earnings, and the earnings will be reportable as income for 2020. The custodian (i.e. bank etc. where you have the IRA account) is required to handle this; make sure to tell them, or select on the website or app, this is a 'correction of excess contribution' or words to that effect, since this is a special case and you need them to report it on 1099-R as such; similarly when you contribute to the trad make sure it is designated for 2020 (this is much more common and routine). Technically you have until Oct. 15, the extended deadline, to do the Roth withdrawal but only until April 15 to do the trad contribution; in both cases I would not wait to the last minute because if there is any delay or glitch you're screwed.

(Note if you have any existing trad IRA, doing the backdoor is less effective; you have to pay tax on part of the conversion. That is regardless of the accounts used; putting the backdoor in one account does NOT isolate it from other trad account(s).)

However, a better solution is to recharacterize the Roth contribution to trad. If you already have a trad IRA, you can do this by transferring the $6k-plus-earnings; contact the custodian (both if different). If you have only the 2020 money in that specific Roth account you can simply redesignate the account as trad, but from your description this doesn't sound like the case. (If you do/did, don't call this a conversion. In the IRA context a conversion is a very specific thing and NOT what you want at this step.) Otherwise contact the Roth account custodian about creating a trad account and then transferring to it as above.

The links to the website are to the 2019 version of pub 590A because 2020 isn't out yet, but these provisions don't change. (There was a change in 2018 due to TCJA eliminating the former ability to recharacterize after conversion, which you see described in this pub as 'new'.) You can also download this in PDF; see the links to the parent pages at the top of this page.

Congratulations on making too much money :-)