It would require either a regular majority of the Commons voting through a motion of no confidence in the government or a qualified majority (i.e. two thirds of MPs) voting for an early election. These provisions are set out in the Fixed-Term Parliaments Act 2011 (FTPA).
In practice, this is incredibly unlikely given the government's current majority. Forty members of the Conservative party would have to vote against their own leader.
Alternatively, parliament choosing to revoke or revise the FTPA would be a possible route to an early election. However, this is equally unlikely to offer Keir Starmer the ability to force an early election. As unless provisions were made to give the leader of the opposition some means to call an election (which the government majority would never allow), it's likely choice of when an election is held would return to the Prime Minister.