There sure is a little round thing on top of your Powershot that has letters like "Auto" "P" "Av" "Tv" "M" and "C" (or maybe not as many but some of these) there, plus a couple of icons.
When you got to the "Tv" mode, youd should see something in your display that indicates a time (and has a green arrow pointing to it?). With the round thing on the back (always assuming that your sd450 is quite similar to my A95?), you can switch on the flash (top), go to "macro" mode (flower icon) (bottom), or left and right. Use the last to change the time.
Tell me if your Powershot IS similar enough to mine so that my explanation helped you.
And be welcome to ThePhotoForum!