How was lenin able to return from exile

The German government helped him return home.
Revolutionary activity broke out in Russia in February, 1917 -- during the time of World War I.   Germany and Russia were on opposite sides in that war, and the German authorities hoped that the return of Lenin to Russia would help foment further revolution there and hurt the Russian government's ability to wage war in Europe.  They were correct in that assumption.  The major revolution in Russia, known as the Bolshevik Revolution, occurred in the fall of 1917.  In March 1918, the new Soviet Russia government ended its participation in the war, signing the Treaty of Brest-Litovsk.