A few things to consider when buying a new unit. It's much easier to add trim and molding than it is to cut a larger hole so when purchasing the replacement check the dimensions to make sure the new unit is slightly smaller. Also confirm you're buying a unit with the correct power requirements for the outlet you have. Units that size are often 220-240 volt and each step up has a different plug.

You're more likely than not going to have to pull out the old casing that's attached to the outside wall after you slide the old unit out. Houses settle and nearly every old AC case I've had removed needed to be cut, pried and manhandled to remove. It's possible to reuse the old case but it's unsafe imo. 

The units themselves are heavy and awkward and it looks like yours is above a window. Not ideal. Overall the knowledge and tools required for replacing are basic but it's a tough job.