i applied the spray and it returned to work normally but after 2-3 days the problem comes back.

In order for that stuff to work, you have to get it on the actual contacts. If you didn't open the camera to get to the button, you probably didn't really clean the contacts effectively. I don't know what the button itself looks like, but many small switches like that are enclosed, so even if you did open the camera it might be hard to really clean the contacts.

Some other possibilities are:

• The cleaning action moved some dirt, allowing the switch to work better, but it's since moved back.
• There's a deeper problem like corrosion or arcing that has caused the problem to recur.
• The problem isn't actually the switch, but perhaps a loose connection between the switch and board, or something like that.
• The cleaning was effective, but caused a new problem such as causing the switch covering to deteriorate.

Your options to fix the problem, ranked from least expensive to most effective:

• Keep trying to fix it yourself. (This may actually be the most expensive in the long run.)
• Take it to a local camera shop.
• Send it to Canon for repair.

There are a bunch of videos online where people take DSLRs apart... if you're thinking of attempting the repair yourself you should watch some of those to get a feel for how the parts are likely to go together, where the screws are, etc. If you can get hold of a repair manual, that will of course help a lot.