Currently it isn't possible to replace a numeric dimension by a symbol or name.

There are a couple of options:

i) You can place a symbol or text before, after, above or below the numeric value. To do this, select the dimension. On the ribbon bar that appears you will see a few active buttons. If you hold the mouse over the penultimate one should say 'Prefix'. Click on this to get the Dimension Prefix dialogue box.

ii) Another alternative is to modify a dimension so that the value isn't seen, and then use the text box to add a label to it. To do this, select the dimension you want to modify, then right click while still on the dimension to get to the Properties. Go to 'Text' and set the text size to e.g. 0.01, then go to 'Terminator & Symbol' and set it to e.g. 1000 time the font size. This then gives you a dimension with arrows but no visible numerical value.