Renaming column names from a data set in pandas

I am trying to rename column names from a DataFrame that have space in the name. DataFrame (df) consists of 45 columns and the majority have spaces in the name. For instance: df.column.values [1] = 'Date Release', and the name should be changed to 'Date_Release'. I tried DataFrame.rename () and DataFrame.columns.values[] but did not work. I would much appreciate it if you could help me to find out what I did wrong

for colmns in df:    if '' in colmns:        colmns_new = '_'.join(colmns.split())        df = df.rename (columns = {"\"%s\"" %colmns : "\"%s\"" %colmns_new})       else:        print (colmns)    print (df)

or this one:

for i in range (len(df.columns)):    old= df.columns.values[i]    if '' in old:        new = '_'.join(old.split())        df = df.columns.values[i] = ['%s' % new]        print ("\"%s\"" % new) print (df)

Error: AttributeError: 'list' object has no attribute 'columns'