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.
On the iPhone or iPad, you may need to display this field.