I have a document in which the spaces between paragraphs are something like 150%. How could I change that for an entire selection to be 100%?
The quick and dirty way is to just select all the text you want to change and adjust the space after from the Paragraph panel. This'll override any existing styles and your text will still update any other changes to your style definitions but you won't be able to adjust that changed spacing globally without selecting the text and manually adjusting it again.
That's not a good habit to get in to though...
What you should be doing is working with well defined styles; both paragraph and character styles. You can then define 'based on' styles when you want a similar linked style with small adjustments (e.g. different spacing). This works well as you only need to update the parent style and the 'based on' (or child) style will update too... except for the extra adjustments that you set on top of the base style.
Take the following spread as an example; the left is using my normal paragraph styles, the right using a 'based on' child style of each, set with extra space after the paragraphs...
It's not clear from your question, but if your extra space is from double line breaks then you can do a find/replace to replace them with a single break. There is a preset query for doing this already; just select "Multiple Return to Single Return" from the "Query" dropdown...
If you don't have the preset for some reason (I'm not sure if it's available in all versions), just find
~b~b+ and change to
\r from the GREP tab.
The proper way to do this is to edit your text and remove any repeating paragraph returns (manually or via find/change if there are many). These can become visible on screen by hitting Ctrl+Alt+I so you can actually see what you're doing. Then apply a paragraph style and play with the 'Space After' property under 'Indents and Spacing'.
There is another option for resolving your particular issue, using the Find/Change dialog box, but it may be hidden for you.
Click on this button, and you will see a couple of additional options that allow you to find and change formatting.
At the upper right corner of the "Find Format" box, is an icon that looks like a magnifying glass with the letter T. Click on that, and it will open another window that allows you to specify what formatting you want to search for. This window is similar to the window that allows you to set options for styles.
Let's say your paragraphs are set with a Space After of 12 pts, and you want to change this to 6 pts. In the Indents and Spacing section, you will set the Space After dimension to match the measurement that you want to change-in this case 12pt. Then click ok.
Click on the same icon for the Change Format section, and set the Space After measurement to 6pt. Click ok.
Make sure that your Search setting is set to the correct option (the entire document, only the story where your cursor is located, etc.), and click on the Change All button. All of the paragraphs that currently have Space After set at 12 points will now have the space after set at 6pt.
However, I completely agree with the other answers that have suggested that you make the change with styles. Many of the coolest tricks in InDesign are built upon the paragraph style functionality--and you are really doing yourself a disservice whenever you make changes using manual formatting overrides. I usually create a "BodyText" paragraph style for an individual document, and then base all of my subsequent styles on this style. Sometimes I will create multiple "BodyText" master styles-one for regular body text, one for text in tables, etc. But, if all of your styles are based on just one or two master styles, you can make a change like what you need to do by quickly adjusting the definitions of the master styles themselves.
A way I just found - select the paragraphs in question and apply a Paragraph Style, which has a single line break.
But I leave the question open for improvement, because that method has limitations when dealing with long text with multiple Styles applied. What is needed is a neat find/change function which will leave the rest of the text formatting (fonts type, size etc.) unchanged.