Face swap effect

That task may be more suited for After Effects, depending upon how much movement is involved. 

Either way, take Safeharbor's advice, as there may be a bit of a learning curve.

Shiraz: This topic was posted on Quora: How do I change face in a video? - Quora

Perhaps you'll find some easy answers there.

Is the face a still image or video clip? Can you maybe post an example screen shot for reference?

For stills, one would edit the still image "face" in Photoshop, making background transparent (alpha channel) and save as .psd or .png format, something that preserves alpha channel transparency. Then in Premiere, just put that image on track above the video clip to superimpose it on background. Adobe Motion effect, with keyframing, could be used to manually track a moving subject. but that can be tedious work.

Thanks

Jeff