You can mark the strings edited in the Default Reference Language to track them in the other languages in three ways.
Update the language using the import functionality
The import functionality (both the one in the project page, and the ones in the translation pages) features a checkbox when you import translations, mark corresponding translations from the other languages as fuzzy for the updated values. Check this box to mark the translations in the other languages as fuzzy, so translators know they need to be revised.
The steps are described in more detail here.
Update individual strings in Terms
Go to Terms, search for the string ID corresponding to the value to be changend in the Reference Language, then hover your mouse over it. Click the T icon (translations) and edit the value in the Reference Language. Then, click on the string ID and then on Save. (Don't change the label, and leave the box under it that says Set all translations as fuzzy checked.) This will put a fuzzy flag on the translations in all the languages.
Update the language using the API
You can use the Fuzzy flag with the API as well, by setting the fuzzy_trigger to 1, as stated here.