How to change a layer in layermap file when stream in

I am not sure what you mean,

The layermap is a text file with explicit translations between GDS layer and how they are understood by cadence in your layout.
In your case, you have:

met1            drawing         74            0

So this mean the layer identified as 74 in your GDS will be translated as met1 in your cadence view
If you place the following line in your layermap:

met0            drawing         40            0

you are now changing the GDS layer 40 into met0 in Cadence layout view

If you want to replace the met1 with met0 in your layer map you simply need to change your strmlyr.in file into

#strmlyr.il
#
#Cadence layer    Cadence layer    Stream layer    Stream data
#name             purpose          number          type
#
#met1              drawing          74              0
#met1 replaced by met0
#
met0              drawing          74              0
met2              drawing          157             0
layer1            drawing          13              0
layer2            drawing          65              0
#
#end of strmlyr.in

If you want your met0 to be both layer 40 and 74 replace the met0 line with:

met0              drawing       40,74           0

Thanks.

It works

Thanks.

It works