I assume you have brought the model in via the bridge?
Once it is loaded, select in the properties panel, then hit “Symmetry”. Wait a while until the symmetry panel turns blue - you should see a perpendicular green frame around the figure, showing that the symmetry plane is in the X- axis. If not, select the X-axis.
Go into vertex select mode and select “soft selection” in the properties panel and select a vert - some verts will turn red. Adjust the strength and smoothness so that the red verts cover the area you want to move - don’t select right on the centre line. Now move the vert you have selected and the others will move proportionately.
Hope this explains what you want - if not, ask away:)