The electoral college will meet on 14 December 2020 and vote. They are widely expected to deliver a result showing Biden has won the election.
However, individual states may have their Electoral College Certificate of Vote challenged when Congress meets in Joint Session to count the electoral votes cast for President and Vice President on 6 Jan 2021. This will be the newly elected Senate and House of Representatives, following the 2020 election.
A "formal objection" must be made in writing and be signed by at least one Senator and one member of the House of Representatives, per the provisions of 3 U.S.C. §15.
In 1969, a challenge was made to attempt to overturn a vote of a faithless elector. In 2005, Representative Stephanie Tubbs Jones of Ohio and Senator Barbara Boxer of California objected to Ohio's electoral votes, alleging "widespread irregularities". Neither of these challenges succeeded.
An objection was also attempted in 2016 by a member of the House, but no Senator signed on.