You can clear cached lyrics by deleting the contents of the Lyrics subfolder in MusicBee's InternalCache folder. Where that is depends on what type of MB install you have--"Installer" or "Portable." If you have the "Windows Store" version you're essentially SOL on that score. I run the following *.bat file to clear mine when I choose to; copy the contents into a text file and rename it with a *.bat extension, like "ClearLyricsCache.bat". Be sure to change the "set dir" path to wherever the folder resides in your installation
Echo Deleting all files from %dir%
del %dir%\* /F /Q
Echo Deleting all folders from %dir%
for /d %%p in (%dir%\*) Do rd /Q /S "%%p"
@echo Folder deleted.
You do know that you can cycle through the providers that do retrieve a song's lyrics manually to find the best match before you save? Go to your Now Playing window and open the Lyrics window in the right sidebar(for example), and with the song active right-click in that pane and choose "Redo Search" to cycle the providers that had a match. Then from the same menu you can choose to embed, edit, or whatever. For my use, in Tags(2) I have both auto-retrieve lyrics and auto-save un-checked. I'm one of those "manual" types, which means I always need to start searching by choosing "Redo Search".
My top 10 providers list order looks like: LyricWiki, LyricWiki Reloaded, Genius, chartlyrics.com, Musixmatch, Metrolyrics, Lololyrics, Urban Lyrics, Song Lyrics, A-Z Lyrics Universe,... and then a few more. The biggest not-available-but-WISH-it-was for me is the Metal Archives site, but I have a web link in MB's "Internet" prefs panel that does a web search there on artist/title that I can then copy lyrics from into the Now Playing panel using the "Paste clipboard lyrics into music file" menu choice.