'TypeError: 'tuple' object does not support item assignment' when iterating through the rows of a layer

I'm trying to assign a value to the attributes of each feature of a feature layer. To do so I am just iterating through each row using 'arcpy.da.SearchCursor'.

#Import geoprocessing.
import arcpy

#Import workspace.
arcpy.env.workspace = r'D:\ArcGIS_Pro_Projects\Optimity_Alf'

#Input parameters
buffer_distance = arcpy.GetParameterAsText(0)

#Input variables.
fc_Opt_All = r'D:\ArcGIS_Pro_Projects\Optimity_Alf\Optimity_Equipment.gdb\Optimity_All'
fc_Opt_Opp = r'D:\ArcGIS_Pro_Projects\Optimity_Alf\Optimity_Equipment.gdb\Optimity_Prospect'
fc_London_Bldngs = r'D:\ArcGIS_Pro_Projects\Optimity_Alf\London_Building.gdb\London_Buildings_Multipatch'
DSM_model = r'D:\Data4Optimimty\ESRI_UK_Bluesky_DSM\ESRI_UK_Bluesky_DSM.gdb\DSM_25cm_2015'


arcpy.MakeFeatureLayer_management(fc_Opt_Opp, 'fl_Opt_Opp')
field_names = ['LOCATION', 'ESTATE', 'ADDRESS', 'OFFSETA', 'OFFSETB', 'Z', 'OFFSETC', 'AZIMUTH1', 'AZIMUTH2', 'VERT1', 'VERT2', 'RADIUS1', 'RADIUS2', 'STATUS', 'CONNECTION']
observers = []
targets = []  
#Calculating LOS for each point
with arcpy.da.SearchCursor(fc_Opt_Opp,field_names) as cursor:
    for row in cursor:

        #Adding default values to observers' attributes
        row[1] = 'Prospect'
        row[3] = 1
        row[4] = 0
        arcpy.AddSurfaceInformation_3d('fl_Opt_Opp', DSM_model, 'Z', 'BILINEAR')
        row[6] = row[4] + row[5]
        row[7] = 0
        row[8] = 360
        row[9] = 1
        row[10] = -16       
        row[11] = 0
        row[12] = 2000      
        cursor.updateRow(row)   

However, I am getting the following error:

Traceback (most recent call last):
  File "D:\ArcGIS_Pro_Projects\Optimity_Alf\Arcpy_Scripts\LOS_Many_Points.py", line 28, in <module>
    row[1] = 'Prospect'
TypeError: 'tuple' object does not support item assignment
Failed to execute (LOSManyPoints).

I really can't see what tuple is the error referring to. Besides, if I do the same thing but for just one feature (not iterating) I do not get the error and it works fine.