Directions are in the manual-- OptiPlex 5050 Small Form Factor Owner's Manual (dell.com)

Thanks! Yep, checked but the manual depicts a regular battery slot, one with a quick release metal tab. My OptiPlex, if you look closely at my pic, seems to almost have a cover preventing you from releasing the battery... 

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I can't tell if tab on right is a release tab or not.  It almost looks like somebody tried prying out with a screwdriver in the wrong spot and broke part of the housing.

I'd try carefully prying it out with a standard jewelry/precision screwdriver, but not where the housing is at full height.

If you had the tab as depicted in the manual, I just use a precision screwdriver against the tab and it pops right out.

Hmm, I can get a better picture but on closer inspection, there doesn't appear to be any damage?

It's almost as if a special tool is required?

No such luck.  I looked at manuals for yours and other 5000 series and they all showed the same battery holder.

This is the screwdriver I would try first.  Precision set of screwdrivers I got from Menard's many years ago.

A couple of things to be careful about.  If you pry the battery, only touch the side of it.  If you touch both side and top of it with a screwdriver, it'll short.  (Top faces MB.)  Or cover screwdriver with Scotch tape.

In your photo, on the very right of the battery, the tab almost looks like a flexible release tab.  May be gently see if it'll move with just the screwdriver and then get the battery out that way.

I do wish Dell would've documented that better.  If I looked for the correct documentation, I probably wouldn't find it except to accidentally run into it when I'm not looking.  : )>  I'm still going to look anyway.

Glad you found that speedstep post and were able to get it.  : )