Using awk:

awk '{sub(/num2=/,"",$4); $4="num2="$4-5; print}' file

This removes the num2= from the 4th column (leaving just a number). Then re-adds the num2= back to the column while subtracting 5 from the number.

The problem is you have done nothing to define RANDOM TEXT so I have a feeling the column you want to work with either won't be in column 4 or won't consistently be in column 4.

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