You can't do that in Picasa. And it doesn't matter at all.
Someone apparently once said "72 DPI is good for the web" and that little "fact" has become some sort of urban legend. The "magic of 72 DPI" is complete nonsense. The DPI value has no meaning for a digital photo. For a digital photo, what matters is how many pixels there are.
On a computer screen, there may be 72 pixels per inch. But most likely, it'll be a different number.
If someone has a monitor that is 24 inches across and they run at 1200x1080 screen resolution, they're seeing about 1200 pixels / 24 inches = 50 pixels per inch (DPI). But if they then run that same monitor at 1920x1080, they're seeing 1920 / 24 = 80 pixels per inch (or DPI).
The size of the photo on a user's screen will vary depending upon the size of their screen and their Windows screen resolution settings. The actual DPI setting of the photo itself is completely meaningless. The actual DPI value of a photo is defined when you finally go to print the photo on a piece of paper. At that point, you simply divide the number of pixels by the number of inches of paper to get DPI. Any DPI value that a digital photo may have is meaningless. At best, it's simply a suggested printing setting. Just about any program is going to completely ignore the DPI setting and print those pixels on your paper.
For example: Let's say you have a 1800 x 1200 pixel photo. It doesn't matter if that photo says it is 300DPI or 72DPI or 3000 DPI. The photo is 1800x1200 pixels. If you print that image on 4x6 paper, you will have a 300 DPI print. 1800 pixels / 6 inches = 300 pixels (or dots) per inch. You could print that exact same digital photo on 12 x 8 paper. You would then have a 150 DPI photo on paper (12 inches / 1800 pixels = 150 DPI). Yet the photo's DPI attribute might be any number at all.