That is totally up to you ... but if you need to make a 1st quarter payment by 4/15/20 then allowing the IRS to keep all or part of the 2019 refund to satisfy this requirement is not a bad thing to do... saves you the effort to make the payment later.
What? Are you at the end and it's asking if you want to apply the overpayment to next year? That means if you want the IRS to keep your refund and apply it to NEXT years 2020 return as an estimated payment. If you want to get your refund now say No.