Many different file types use stm extension and it is likely that at least some of them are text-based so you can view them using any text editor. You can simply rename stm to txt extension and check if you can read the contents in Notepad, no converter required.
Convert LloydsTSB bank statement file to Simple text file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert stm to txt on Windows
A popular spreadsheet application distributed with Microsoft Office suite
Convert from stm file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.