If you have multiple configurations in your model, and edit a dimension that's shared by all configurations, by default that change will affect all configurations. I personally like it that way. If I haven't told the software that this dimension should differ between configurations then I want any changes to affect all of them. Others may disagree. Now that I have that out of the way, there are several ways to edit a value only for selected configurations.
1. One way is to open the standard dimension dialog box for the value, click on the Configuration icon, and make the appropriate choice. If you do this outside of an active sketch then you'll probably need a manual rebuild for the change to take affect.
2. Many people use that method, but I prefer another workflow that doesn't require me to be in "Edit sketch" or "Edit feature" mode. In fact, it only works when there isn't a feature or sketch active. I single-click on the feature or sketch in the tree to display the values. (If you don't have Instant 3d turned on then you'll need to double-click on the feature, or go to Annotations at the top of the tree and choose "Show feature dimensions".) I then right-click on the dimension in the graphics area and choose "Configure Dimension" from the drop-down.
This will open up a simplified design table, with a column for the configuration names and one for the selected value, and a row for each configuration. You can enter the desired value in the appropriate cell for each configuration in this table.
While this table is open you can double-click on other dimensions, sketches, or features to add columns. If you double-click on a sketch there will be a drop-down where you can select sketch dimensions to add.
As you can see at the bottom, you can also create new configurations from this table. I often use this method to create new configurations instead of going to the configurations tab and adding them there.
If you'll enter a name for the table at the bottom you can save it for future edits, but the changes you make in this table will still be saved even if you don't save the table. And as I said above, after closing this table you've effectively "told" SW that this dimension is configuration specific, so if you want to edit the dimension later you don't need to re-open the table. You can just click on the feature to show the value in the graphics area again, click on it, and change it to the new value. That change will only affect the active configuration, and won't require a rebuild for it to take effect.
2019-03-07 edit: My wife and I (mostly her) are having some remodeling done, and because of that the TV is unhooked, so I'm sitting at my laptop browsing through some of these blog posts, and it occurred to me that I should give some credit. I learned about this option from Paul Marsman at question about linear patterns and configurations.
3. A third method is to set up an Excel-based design table by going to Insert > Tables > Design Table. That will open a property manager where you'll have a number of options. Below is the result for this Part when I chose the default settings, including "Auto-create". As you can see, it adds a column for every dimension and custom property (I highlighted the column for the same dimension I used in the methods above). If you don't need or want that many columns you can choose "Blank" instead of "Auto-create" and only add the parameters you want, or of course delete columns from the auto-created table.
I don't use the Excel-based design tables much, so if you have a specific question about them I'd encourage you to post the question in the forum instead of in a Reply here, and I'm sure someone will be glad to help. By the way, I know people follow the forum from several different places, but I generally do it from the All Content page. It's very simple to start a new Discussion from there.