[oki270]

Let's say I take one song (for instance - Clapton's Tears in Heaven) and I would like to make it sound more like jazz song. Are there any available rules how to do it or it's just trial and error?

O.

[allthumbs]

Use jazzy passing chords, augmented and diminished. Substitute jazz chords where you can. 9ths,11ths and 13s.

[scotty_b]

There are so many options. Marty's response is right on the money.
Jazz will typically use extended and altered harmonies to create more depth in the tension and resolution of the piece. Passing chords, voice-leading, chord substition, the importance of the bass line, all play a part.
This is a very big topic, but perhaps one place you can start is by taking a melody note from the song, and working out any possible chords that can use that note. Place the melody on the top of the chord, where it will stand out more readily.
If you are interested, here is an arrangement of Amazing Grace that I put together Amazing Grace jazz chord-melody approach