The below query would help if want to convert short month string (e.g) Jan, Feb to full month string (e.g) January, February...
Assuming mth_shrt in table1 has value like Jan, Feb, Mar and after the execution of the below query the result would be
Result:
January
February
Hope this helps. Post your views on this.
select datename(month,convert(varchar(20),'01/'+ mth_shrt_nm + '/1900',121)) from table1