Not sure which reports you've been reading - he's looking overwhelmingly likely to be the Republican nominee at the moment but whether that'll still be the case if he gets convicted in one of the many trials he's facing, remains to be seen. It's also by no means certain that he'd defeat Biden come November.
Again, whether he's been convicted in the meantime (and there's no guarantee of that) will make a big difference, with 45% of Republican voters saying they would no longer vote for him if that's the case, and even more if he actually goes to prison.
52% of Republicans won't vote for Trump if he goes to prison