How this could happen without chucking out the Constitution and using a wholly unconstitutional process is not clear.
To be constitutional, Biden and Harris would have to be impeached and convicted simultaneously so that no replacements could be confirmed, while Trump was selected as Speaker of the House so he would be in line to take over the Presidency.
That latter part is at least conceivable, if Trump and the Republicans believed this method would work. But even they have to know that there's no way the Senate Democrats would ever convict Biden and Harris, and absolutely not simultaneously so that they knew they were giving away the White House to any Republican, much less Trump.
So they have to vaguely be imagining some procedure that supersedes the Constitution, such as a federal judge issuing an arrest warrant for Biden and Harris, the US Marshalls or MPs perpwalking them out of the White House, and - probably without going through the process of becoming Speaker of the House - Trump just being told by some federal authorities to go in and take the oath of office (no doubt on a God Bless America Bible).
And they would undoubtedly insist that this is more constitutionally valid than allowing Biden, whose electoral win was validated by Congress in due procedure, to stay in office.