You can edit source strings in Transifex for file formats that use key-valued pairs. This is explained here.
For file formats that do not use key-value pairs, you can add a new target language as an intermediary and make your edits in that language. For example, if your source language is English (en), add English (United Kingdom) (en_GB), then use your edited strings as the "translation" for English (UK).
When you're done with the edits, download/pull the "translations" from Transifex, delete the existing online content, and upload/push the "translations" back to Transifex as English (en). From there, you can translate the strings to your target languages.