How can I convert files when my password is not recognised, using Ubuntu 18.10 OS?

I’m a beginner! I only have one password, and this doesn’t seem to be recognised - please can you help?

If you don’t remember the password, there’s no way to open the file, and thus no way to “convert” it. The only way is to recall the correct password.

I did answer to Mike - I haven’t forgotten my one and only password!!

If you are sure that you use a correct password - then you need to provide some more information.
Where have you set the password - specifically in which program? Was that LibreOffice? Or some other application? If LibreOffice, then what happened now that it refuses: did you upgrade it? If so, does it help to downgrade? Which version of LO do you use now to open the file? Does it help to upgrade to newest version (6.3.0)? Did you file a bug report on that, providing a sample password-protected file created in an older version/another application, that can’t be opened in current LO?

Sorry about the confusion. I have just had Ubuntu 18.10 (64-bit) downloaded to my desktop by my local PC shop (as a dual-boot with Win7 - as I want to get rid of Win as soon as I can), I then updated Ubuntu to the latest level. This included Libre Office Writer 6.1.6.3, which I thought was part of the package. When I asked the technician what my password was to convert docs in Libre, as well as the one I must use when downloading something. He told me that they would always be the same as my Ubuntu password to log in.
Was he incorrect?

Thank you,
Ashley

Oh. So the initial question should have been then: where and when do you see a prompt for a password? Can you edit your question to clarify it - and move all the clarifications from here (the “answer” that is not answering your question); add a screenshot of the password prompt?