How can I adjust nth number in a line?

Using awk:

awk '{ for (i=1;i<=NF;i++) { if ($i ~ /num2=/) {sub(/num2=/, "", $i); $i="num2="$i-5; print} } }' file

This will loop through each column of each line looking for the column that contains num2=. When it finds that column it will:

1. Remove num2= - sub(/num2=/, "", $i)
2. Redefine that column as num2={oldnum-5} - $i="num2="$i-5
3. Print the line - print

perl:

perl -pe 's/(?<=num2=)(\d+)/$1 - 5/e' file

To store the contents back into the file:

perl -i -pe ...

[Just because I'm trying to become more familiar with Miller - it requires some jumping through hoops to get the heterogeneous output]:

$ mlr --fs ' ' --repifs --ocsvlite --headerless-csv-output put '
    $num1 = "num1=".$num1; $num2 = "num2=".($num2-5)
' file
RANDOM TEXT num1=400 num2=10 RANDOM TEXT
RANDOM TEXT num1=300 num2=5 RANDOM TEXT
RANDOM TEXT num1=200 num2=0 RANDOM TEXT

If the input is actually TSV in which some columns may contain spaces, then

$ mlr --fs '\t' --ocsvlite --headerless-csv-output put '
    $num1 = "num1=".$num1; $num2 = "num2=".($num2-5)
' file
RANDOM TEXT num1=400    num2=10 RANDOM TEXT
RANDOM TEXT num1=300    num2=5  RANDOM TEXT
RANDOM TEXT num1=200    num2=0  RANDOM TEXT