Can I set thresholds for 74HC14N (6 NOT gates)? and use as schmitt trigger
The thresholds for the 74HC14 can not be modified externally from the chip.  The thresholds actually vary quite a bit, as shown  in the following table from the datasheet:



With a 6v supply, it appears the maximum low threshold VT- (3.2v) can supposedly be greater than the minimum high threshold VT+ (2.1v), however this is actually overridden by the last parameter VT+ - VT- which guarantees the minimum separation of 0.5v.  However as one can see, there is not much control over these values.

However you can add a precise amount of hysteresis to a comparator using the following circuit:



where V2 is the high threshold value (turn off) and V1 is the low threshold (turn on).

The resistor values are calculated using the following relationships:

$$\frac{R3}{R1} = \frac{V1}{V2 - V1}$$

$$\frac{R2}{R1} = \frac{V1}{Vref - V2}$$

The calculations are made much simpler using this online calculator.

And here are some additional circuit notes from TI about adding hysteresis to comparators.

If one doesn't have a comparator available, an op-amp can be substituted, but it is not as satisfactory as discussed here.  Nevertheless, the datasheet for the LM324 even includes a circuit for using it as a comparator with hysteresis:



quite similar to the one shown earlier (just no resistor from the + input to ground).