How to convert galvanic cell into electrolytic cell? 

 Conversion of a galvanic cell into an electrolytic cell is practically  possible  by  -

(i) connecting the cell to an external source of electricity , and

(ii) providing enough  potential difference through this external source  ,   such that  ,  the applied emf  exceeds  the  emf  of  the galvanic cell  itself. [ for example  >  1.1 V   ,in case of   simple Daniel cell  ie.(  Zn - Cu ) -cell. ]

Explanation :

Thus , In the   beginning the external opposing potential is less than 1.1 V.   In this case the normal cell reaction  occurs in the galvanic cell  and the current flows from copper electrode to zinc electrode. But when the opposing potential is  increased slowly and gradually it is observed that normal reaction continues to occur till the opposing potential reaches  to value of 1.1 V. At this stage the cell reaction stops .& the current continues to flow . Further increase in the opposing potential ( beyond 1.1 V ) again causes the chemical reaction to occur. But now  the reaction occurs in opposite direction forcing  Zn²⁺  ions to get reduced and Cu atoms to get oxidised.

Zn²⁺ (aq)   +     Cu (s)  ---------->     Zn (s)     +      Cu²⁺ (aq )