You can just simply rename SRT file extension to TXT, because much like many other subtitle formats, these are but a specially formatted text files readable by any text editor, so no srt to txt conversion is actually required.
In case you manually edit and save .srt files, pay extra attention to the text encoding of the subtitle file. Sometimes text can be encoded in different character sets such for example UTF-8 (universal for many languages) or ASCII (simple English text) or many other code pages for different regions and countries / languages.