Even if you could prove fraud in every state....
The constitution gives no figs for the popular votes of the states, only the electoral college. Did the electoral college vote for Biden? Yes. His presidency is legitimate.
Thus it is as
@edgeworthy and
@Husky_Khan say. The most extreme result possible would be an impeachment on the grounds that the current president and vice president gained their positions via fraud, followed by Nancy Pelosi becoming president.
Long term there's likely to be more reprecussions. Laws protecting the electoral process are going to be passed, and anti-fraud measures are going to become more stringent. It's quite likely there's going to be mandatory audits for every election from school president on up going forward. Trump has a fair chance of winning in 2024 if he can campaign on "I actually won but was defrauded." Public knowledge that they cheated is going to hurt the Democrats... a lot. They will lose scads of voters who want to see themselves as being in the moral position. Most likely the Republicans win the next 3-4 presidential elections and rake in legislative seats while the Democratic party disintegrates entirely. Bereft of any competition, the Republican party gets complacent and stupid and proceeds to self-destruct as well, pushing too hard when there's no enemy to push back and promoting laws that are mostly pork and grift once they get too comfortable in their ways.
After a few cycles the five dozen parties that result will distill down into two major parties again, because the US voting system can only support two major parties plus relatively unimportant fringe groups, this is mathematically proven. A significant number of core positions remain the same, while some edge positions may swap sides due to deal-making and how things shake out. You may find hardcore environmentalism on the conservative side this time, f'rex, supported by enthusiastic hunting and fishing advocates who want nature to be preserved for their lifestyle; while the left gains a greater focus on private charitable groups in the name of doing good.
This will, unfortunately, be heavily influenced by corporations picking who to fund, though most will simply fund both sides as usual and reap the benefits no matter who wins.