You want to exchange the item you purchased. That is, you want to return to the store the one you bought and have the store give you a new one for one reason or another.
You want the store to replace it.
The clearest way to express this in American English is to say to the salesperson or service desk person:
I'd like to exchange this rug for a larger one (because it's too small for the space I want to put it in). [NB: The reason is optional unless you're asked for a reason.]
I'd like to exchange this space heater for a new one that works. When I tried to use this one that I bought here yesterday, it didn't work. Can we test the new one here before I take it home?
"Mandatory" means that the thing must be done due to some reason or rule.
These courses are mandatory.
That is, you must take the courses or else you fail.
Wearing helmets was made mandatory a few years ago.
It is a rule that you must wear helmets.
"Indispensable" is that the thing is so critical, useful, or important that you cannot conceivably throw the thing away.
The volunteers' help was indispensible.
The mission would have failed if the volunteers didn't help.
He made himself indispensible to the parish priest.
He was deemed a must-have person because of some good quality.
These great tools are so versatile that they are indispensable.
The tools are so good that you shouldn't be without them.
To sum up, these two words are not interchangeable - they have their own distinct meanings.
Best Answer
Modify does not necessarily mean changing something for the better - just changing in general. The other two words imply more strongly that whatever it is you're amending or revising has some problem which needs to be fixed.
As for the difference between the other two - they're similar in meaning, but amending something is generally a shorter process focused on singular issues, while revising is more involved and typically means you've re-checked everything and made changes as necessary.
So in short: