The green background that the designer used. Is it a bitmap? And if so, what is the resolution of the bitmap?

If the image is going to be displayed on the web then 96dpi is OK. If the image is going to be used for commercial printing, then it has to be 300-600 dpi. You cannot make an image that is low resolution, high resolution unless you physically reduce the image but about 1/3 or more which makes the image smaller but increases the resolution. If you go the other direction, that is, make the image larger, the resolution will decrease proportionately and the image will become blurry.

My question is, as you are not experienced in design, do you want to be making changes to something that was created in Illustrator in Xara? And why don't you have the designer who created the work in the first place make the modifications?