In bash if you have the string in a variable, you can try parameter substitution
to remove everything after the sign =, i.e.
this in practice substitutes the = sign and the rest of the string with an equal sign alone. On the other hand if you want to change the content of a file the solution is sed (as you guessed). Something like this should do the trick:
sed -i '/hellowolrd=/s/=.*/=/'
which is practically the same as above.