They will be challenging the mail-in ballots which will favor Biden. They are doing it already, suing over state laws governing mail-in ballots.
Areas that they are challenging or will challenge:
Unsolicited ballots. Here in NJ, all registered voters were automatically mailed ballots to encourage mail-in voting rather than voting in person. According to Trump, unsolicited ballots should be illegal.
Drop Boxes. Texas only allows one per county. They want to make it difficult for voters to vote by mail. That law was struck down by a lower court but upheld by an appeals court that was packed with Trump judges. Here in NJ, each municipality has its own drop box so that it is easy for voters to vote by mail.
Cancellation stamps. Some states, like NJ, provide postage paid return envelopes. Postage paid envelopes are not always cancellation stamped so Trump will challenge them in terms of proving when they were received.
Receipt dates. Some states allow mail-in ballots that are received after election day to be counted as long as they were mailed and cancellation stamped by election day. This is already being challenged. Trump wants only mail-in ballots that are received by election day to be counted.
Signatures. Signatures will be the "hanging chads" of the 2020 election. Any ballots without exact match signatures will be challenged. Voters do not update their signatures which change as they age or due to illness.
These are just a few of the areas that will challenged. One of the interesting results of this election will be seeing what legal challenges Trump will make. I'm sure that his legal team will be extremely creative.