How do I replace the 1tb hard drive with an ssd

You have a few different issues here you'll need to address:

• The SSD you are getting will need to be a 3.5" mount most are 2.5" or you'll need a converter bracket set to mount the 2.5" SSD unit.
• The '11 iMacs use the internal temp sensor of the HD to manage the fan speed. If you disable this your system fans will go into full speed as it can't access the sensor. There is a solution here, you need to get from OWC the special thermal sensor unit. Some newer SSHD/HD's now work with the Apple sensors so you can swap the drive, but the SSD units don't offer the Apple SMC integration to manage the fans.

Frankly, I recommend swapping out the HD with the correctly sized SSHD unit (hybrid drive). These drives have a SSD for a deep cache gaining the benefit of the speed of the SSD but giving you the lower cost of a HD for the same sized SSD. The latest versions offer 4TB and 5TB units are just coming out now. That is if you want the easiest upgrade path.

A dual drive setup would also be a good direction here as well. As this system was designed with a dual storage solution (not loosing your optical drive) is a great upgrade too. Once you have the SSD installed you can then create a Fusion Drive marrying it to your current HD.

We've done both and love both. The issue is what your needs are and what you are looking for. The good: speedy boot ups & faster app loading. The bad: complexity, costs, & limited in-system of storage space.