This McClatchy piece goes through a few good examples of concrete policy differences. The short version is that because the Democratic Party shifted leftward between 2016 and 2020, Biden's election platform tended to be more liberal than Clinton's was in the 2016.
On health care, Clinton proposed offering a public insurance plan for
Americans enrolled in the health care exchanges established by the
Affordable Care Act. She also wanted to let adults older than 55 buy
into Medicare. Biden’s plan goes much further: He wants to allow all
Americans — including those receiving insurance through their employer
— to buy into a government-backed insurance plan, a shift some
progressives have said would represent an enormous change to
Obamacare. (Biden also proposed significantly increasing the subsidies
available to those who enroll in the public option.)
There’s also a wide disparity between Biden and Clinton’s climate
change plans. Clinton proposed spending $60 billion on clean-energy
fund as part of an attempt to make the U.S. 80% carbon-free by 2050;
Biden wants to spend $1.7 trillion in federal money to make the
country emit a net of zero carbon emissions by 2050.
Biden is also pushing to triple Title I funding for schools that
educate low-income students, and to abolish the federal death penalty
while encouraging states to do the same. Clinton wanted to preserve
capital punishment in certain situations.
It's also been argued that Clinton is in general more hawkish than Biden based on the foreign policy stances they took when they were both in the Obama administration. Biden opposed the Afghanistan troop surge, the Libya military intervention, and sending military aid to Syrian rebels, while Clinton supported them all. Most famously, Clinton supported the Bin Laden raid while Biden suggested the U.S. needed more intel.
But if you're asking for major policy differences like the sort of policy differences between Clinton and Sanders, or Biden and Trump, you're probably not going to find a good example. Clinton and Biden's platforms differ in a manner of degree and urgency, but they occupy the same lane of the Democratic Party and share basically all the same policy goals and foundational principles.
(And quite bluntly, both Clinton and Biden did a good job of, ahem, evolving away from whatever opinions differed from the party's center before they ran for president. In 2013 you could've said that Biden supported gay marriage and Clinton opposed it, and in 2016 you could've said Clinton supported taxpayer funding for abortion and Biden opposed it. But both candidates shifted their views by the time they ran for president.)