Hello Amaresh,
Links to images can be relative, but links to Excel use Object Linking & Embedding (OLE) technology. Unfortunately, OLE *insists* on full paths; it doesn't permit relative paths.
Assuming that your PowerPoint file and any linked Excel files are on a local or network hard drive (ie, NOT in the cloud/in SharePoint/etc), what *should* happen is:
PowerPoint will look first for the linked file along the full path to the file that's part of the link.
If it doesn't find the file there, it will look in the current directory (ie, the same directory/folder as the PowerPoint file is stored in. Usually.)
There's one oddity to this in some (all?) versions: if you open the PowerPoint file in nearly any way (doubleclick its icon, choose File | Open etc) PowerPoint will set the current directory to the directory the file's in. BUT in some cases if you use the recent files list to open the file, the current directory doesn't get set and will be something random.
So basically, if you need to update the linked Excel data and still retain a copy of the old version:
Make a backup copy of the original (say, with the date as part of the file name), put the original in the same folder as the PowerPoint file and edit away.