The second can be done by warming or cooling the ocean. Colder water is denser, so the same mass of water occupies less space. In considering sea level changes, an important consideration is the rate at which they occur. 1 meter in 1 day is quite disastrous. 1 meter in a million years would be irrelevant on the human scale.
Water has a small but nonzero expansion as it warms.
The expansion is approximately 2E-4 per degree of warming, at the temperatures of the upper ocean. To convert that into a sea level change, we need to multiply by the amount of warming and the thickness of the ocean that gets warmed. The amount of warming is the subject of the climate modelling. Let's consider a warming of 1 K for simplicity. The central question for the oceanographers is then how deep a layer of the ocean gets warmed.