My 1990 civic just had the engine to a 1998 put in it...
Lately the engine has been revving high...
If I pull back on the gas pedal the rpm returns to normal like its not returning to original position placing tention on the cable...
How can I fix this

Make sure the return sprigs are on correctly. Make sure the throttle cable is hooked up right and there are no kinks or anything holding it up.

Adjust the throttle cable--loosen the jam nut a turn and tighten the inside nut against the bracket.

And the jam nut and bracket are located under the hood at the intake manifold correct ?

Yes.