It looks like it’s a bottom up pattern correct? Even if you have to rip some out you shouldn’t have to take out the whole thing to correct it. If it’s too short from the underarm to the bottom you can rip back to below the underarm and then add those 5 rows and finish again. Or you can rip back plus the 5 rows and then finish. It’s a lot of work, but you’ll be happy you did when you’ve got a finished garment you can wear.
It’s a pain to do, but you could rip out the cast on at the bottom stitch by stitch to decrease 5 rows or just rip out the cast on and put the stitches back on the needles and then knit 5 rows and bind off. It’ll look slightly different than the cast on on the other side, but not enough to matter for a tank top especially if you do a border all the way around.