office product key won't work

I bought Office for mac and when I tried to input the product key code, there was one there already and no way to change it out. I bought this used imac and guess that may be the problem. I deleted many many Microsoft files and folders twice, no change. Does anyone have some advice?

How about telling us which version of MS Office for the Mac that is on the used iMac so we can direct you to the uninstall instructions?

I figured it out!! I was not able to edit the product key box until I went to the edit pull down at top of page and selected "select all". It did not select it all, but somehow awakened it so I could input the right key code.

I contacted the last owner and he never had Office on this Imac.

Thanks,
Patt