Making a specific version the default version

I have several versions of sketchup installed in my computer, how can I make a specific one the default version for opening files?

for example I have 7,8,15,16 but everytime i want to open a file it opens it in b but I rather it to open it in 7

I meant to say that it opens it in version 8 and Id like for it to open it in version 7 and when I right click the mouse button to make a version the default version only 15, and 16 show up

That is a question for using Microsoft Windows, not Sketchup itself.
My right click in Windows 7 on a Sketchup file allows me to select the “Open with” option, and appears as follows:

When I select Choose default program in the sub-menu, I see the Open With dialog box

I can select the Browse button and rummage through Windows to choose any program.

You will need to select the desired executable in the “Open WIth…” dialog box. On my computer it starts out in the “Progam Files” folder. That folder tends to contain 64 bit versions of programs. Depending on your version of Windows, you may need to navigate over to the Program Files (x86) folder that has the older 32 bit programs for older versions of Sketchup.