Difficult...
Going to be tricky to get a Conservative win in '45 without a major point of divergence, giving a number of circumstances stacking the deck in favour of a Labour win if the original timeline election takes place after the original timeline victory in Europe.
Simplest way I can think of is something (not sure what) happens during the war which causes Churchill to change parties to the Labour party.
It's not like Churchill hadn't changed parties before...
Alternatively, something causes the war with Japan to drag on (maybe Truman decides not to drop the atomic bombs, and the Russians don't declare war on Japan at their original timeline date???) and although Labour win a clear majority, Attlee decides a national coalition is called for, and invites Churchill to remain on as Prime Minister, on the understanding he confines himself to foreign relations and defence issues*.
Edit:
* Yes, this latter option doesn't involve Churchill actually 'winning' the elections, which the thread title calls for, but the actual opening post, which was what was foremost in my mind in writing this response, simply calls for Churchill to remain as Prime Minister...