How can I Change Ubuntu 14.04 from 64 bit to 32 bit ?
You will need to reinstall Ubuntu using the 32 bit version, make sure you backup all important data first.
Just because you only have 2Gs of RAM doesn't mean that you need to downgrade to 32bit- (In fact, downgrading isn't likely to solve your real problem) Allow me to offer an alternative solution:
Use a lighter desktop environment, such as LDXE or XFCE
If you wanted to try this alternative first, simply run the following in a terminal:
sudo apt-get install lubuntu-desktop
sudo apt-get install xubuntu-desktop
for LDXE and XFCE, respectively.
You see, the default Ubuntu desktop - Unity - is what requires more than 2GB of RAM -- If your machine boots the 64 bit version, that's good - ! - but if it doesn't run very well, that's because Unity wants more than 2GB of RAM, and downgrading to 32bit isn't going to change that fact...
There is also LXQT coming soon, which you should also be able to run with just two gigs of RAM.