There is no button for this particular problem. The solution is, that you have to study what Mp3tag can help to solve your problem.
See Mp3tag Actions and related functionality to replace some character string with another character string.
There are many sources in the forum regarding UNSYNCEDLYRICS.
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DD.20100907.1242.CEST
Edit. Search link removed, because it could mislead.
Possibly this link offers help.
Actions
Also there are functions in the Mp3tag scripting language, which can be used along with an action "Format value" to solve the task.
DD.20100907.1805.CEST