Remove the post using a wrench (adjustable or 7/8"). Make sure the wrench is firmly seated you dont want to round off the corners of the post - you will be removing and reinstalling the post for cleaning. The dip tube should be removable thereafter - if you cant get it out with your fingers you can use pliers but be careful if you do as the rim is relatively fragile and you dont want to deform it as that can cause leaks.

Unscrew the liquid out post and the dip tube can be pulled out

Unscrew the upper piece from the lower. If a tube with a grommet is already there remove them then slide in the new dip tube (use the old grommet on the new tube if necessary)

Looks like 1/2" to me.

it was what was available and did not matter for my application. it was what was available and was not an issue for my application. Besides the thread sizes generally are interchangeable. if you don't like the fit use some teflon tap.