In [90]: df = pd.DataFrame(np.random.randint(10**5,10**7,(5,3)),columns=list('abc'), dtype=np.int64)

In [91]: df
Out[91]:
         a        b        c
0  9059440  9590567  2076918
1  5861102  4566089  1947323
2  6636568   162770  2487991
3  6794572  5236903  5628779
4   470121  4044395  4546794

In [92]: df.dtypes
Out[92]:
a    int64
b    int64
c    int64
dtype: object

In [93]: df['a'] = df['a'].astype(float)

In [94]: df.dtypes
Out[94]:
a    float64
b      int64
c      int64
dtype: object