Thanks Kevin for your profesionnal advice; I am an accountant and I agree. However, under specific circumstances the cancellation can be necessary. It is my case because I misunderstood how OE setup default debit & credit for accounting journals is functionning.

And now I am in a "corner" because I am using the Saas for OE where I can not add community modules.

Finally, I found the button which enables the cancellation of an accounting entry -one by one. Select an accounting entry for cancellation, then select the little arrow next to the entry reference number or code; this opens the accountig entry itself; the click on the grey top left button ; the entry is then set to draft; make corrections if any and confirm/save; or delete the entry.