import arcpy

# Your input feature class
fc = r'C:pathtoyour.gdbfeature_class'

# Start an update cursor and change values from 0 to 100 in a field called "your_field"
with arcpy.da.UpdateCursor(fc, "your_field") as cursor:
    for row in cursor:
        if row[0] == 0:
            row[0] = 100
        cursor.updateRow(row)