Copy and paste between special lines using awk command

Lets assume that the given awk command

awk 'NR==1,/mode/{next}/tot/,NR==0{next}{print}' 1.out > 2.dat

was executed, and created file 2.dat.

In order to modify an existing file 3.out as requested, here is the answer using one awk command (to create a temporary file /tmp/3.out.tmpfile):

awk 'FNR==1{nf++}; NR==1,/mode/{print;next}; f2==0{f2=1;nextfile}; nf==2{print;next}; /tot/,0{print}' 3.out 2.dat 3.out > /tmp/3.out.tmpfile

and one mv command (to replace the original 3.out file by the modified one /tmp/3.out.tmpfile):

mv /tmp/3.out.tmpfile 3.out

For example, if the original 3.out file contains:

line-1
line-2
mode
line-X
line-Y
line-Z
tot
line-3
line-4
line-5

and file 2.dat contains:

inserted-line-A
inserted-line-B
inserted-line-C
inserted-line-D
inserted-line-E

then the above proposed commands will result in a modified 3.out file that now contains:

line-1
line-2
mode
inserted-line-A
inserted-line-B
inserted-line-C
inserted-line-D
inserted-line-E
tot
line-3
line-4
line-5

The original 3.out lines between mode and tot have now been replaced by the ones in file 2.dat.