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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