The acoustic's truss rod is used to counter tension against the strings to keep the neck in the proper position. It is not used for adjusting the action; this is done at the nut and bridge. The head of the truss rod is accessed at the top of the neck or from within the sound hole depending on the guitar.
To check the tension on the neck start by making sure the guitar is properly tuned. Then place a capo on the first fret or use your available hand to do so and at the same time press down the first string at the 14th fret. Now check the clearance at the 6th fret. You should be able to just fit a piece of heavy card stock (index or business card) under the string at the 6th fret. Tighten the truss rod to decrease the clearance. Loosen the truss rod to increase the clearance.
The height of the saddle is the main component in determining the action. It is adjusted by removing material with sandpaper and a sanding block.
The top should be slightly contoured to allow the thinnest strings to sit a little lower. The overall action varies based on preference but a clearance of 3/16” above the 12th fret is a typical rule of thumb. I recommend buying a few acoustic guitar saddles to allow the freedom to experiment and find the best setup.
If you change the gauge of your strings you should double-check the truss rod and bridge. Thicker strings will produce more tension and will need a bit more force to counter it. Also, they require greater force against the fretboard so you may want to lower the action at the saddle to compensate. If you plan to try a new string gauge then you should plan on ordering a new saddle also.
Going through these steps may take some time but it will be worth the effort. Now that you know what to look for, the next time you walk through a guitar shop check these parameters on a few guitars and you will be surprised at how variable the factory setup can be. It will put a smile on your face when you compare the playability of your properly adjusted $80 guitar to some costing several hundreds of dollars but still in need of a little work.