Reductive amination in case of secondary amines
In modern organic synthesis, reductive amination of aldehydes and ketones is considered one of the important and principal ways to make secondary and tertiary amines. It is a powerful and reliable strategy for the formation of $\ce{C–N}$ bonds. One of the advantages of the reductive amination is it can avoid the problem of over-alkylation that is always the problem with direct alkylation of amines with alkyl halides. 

Reductive amination can be achieved following either a one-step or a two-step procedure. In both procedures, an amine and a carbonyl compound condense to form an imine (with primary amine) or iminium ion (with secondary amine). This imine or iminium ion is then reduced in situ (one-step reaction) or separated and subsequently reduced (two-step reaction) to form an amine product. Following is the suggested mechanism for reductive amination:



Most common condition is sodium triacetoxyborohydride ($\ce{NaB(OAc)3H}$) in THF or dichloroethane (DCE) under slighly acidic conditions (common acid used is acetic acid)(Ref.1).   

Reference:


Ahmed F. Abdel-Magid, Kenneth G. Carson, Bruce D. Harris, Cynthia A. Maryanoff, Rekha D. Shah, “Reductive Amination of Aldehydes and Ketones with Sodium Triacetoxyborohydride. Studies on Direct and Indirect Reductive Amination Procedures,” J. Org. Chem. 1996, 61(11), 3849-3862 (https://doi.org/10.1021/jo960057x).