You need to find the right keyframe in the action’s fcurve and change its x coordinate:
import bpy
ob = bpy.context.object
if (ob is not None and
ob.animation_data is not None and
ob.animation_data.action is not None):
for fcurve in ob.animation_data.action.fcurves:
# grab location.z curve
if fcurve.data_path == "location" and fcurve.array_index = 2:
for k in fcurve.keyframe_points:
# find keyframe at frame 60 and move
if k.co.x == 60:
k.co.x = 77
break