Nothing stated above is incorrect davis119. They have shown the OP how to depressurize, remove the spear in order to clean and fill it as the OP requested.
That said, to actually pressurize, reverse the process. Hook up a coupler with a shut-off valve or a penny over the liquid out port (what I do) and then carbonate as normal. Getting the ring back in takes some practice, but after a while it becomes second nature and takes ~30 seconds to install. Use a wrench to re-install the ring; there is a tutorial on HBT here somewhere that shows the ring install process with the wrench.
Sixtel 5.16 gallon Sanke's are great for kegging, and I actually switched from years of using cornie kegs to using Sanke's over the last 12-18 months. Went from 16 cornies to 16 sankes and I have zero regrets, except that I didn't start out with Sankes.