Is your logbook filling in the wrong Hobbs or Tach Times? This can be reset in the Aircraft tab.
Automatically filling the last Hobbs/Tach times is controlled by the specific Aircraft ID entry. It's built to only count upwards, so it sounds like the current value was mis-entered at some time, and needs to be corrected.
Go to the Aircraft tab, select the Aircraft ID with the incorrect entries, and you'll see a Hobbs or Tach field (whichever you need). Enter the correct values, and the next flight with this aircraft will auto-fill the correct value.
If you do not see the Hobbs or Tach fields, you might need to make them visible → see how display this field.)