In calc how do i select background object

I cannot select it, so I cannot bring it to the foreground.

Sorry. My original answer was misleading and incomplete.
-1- Unfortunately the white pointing arrow always looks exactly the same though it has different functionalities. In Calc it selects cells by default. A foreground object where covering the area of cells can also be selected.
-2- Wanting to select background objects you need to first activate the ‘Select’ functionality from the ‘Drawing’ toolbar. This is also needed if you want to create a frame for selection.
-3- To select an object then by pointing is surely simpler then than by catching it with a frame.

(Original Answer:)
Draw a selecting frame completely containing the object. You may even need to change the zoon for the action.

Because of the community’s stupid rules, I can’t get enough points in order to upvote answers although I’ve been a member of this community for the last couple of years. Hence, currently I’m unfortunately still unable to upvote good answers like yours. Anyway, congratulations: your answer did help me solving the problem.

If you ask a question like “How to get entitled to upvote contributions?” someone might upvote that question an you would get “karma”.
From your user page I see that you only posted one answer to date, and unfortunately nobody upvoted it. (I cannot do it in this case because I lack knowledge needed to judge if that answer was good.)

Thanks, this is the solution!

Use the navigator on the side bar or Menu/View/Navigator [F5]

For me (V 5.4.2 currently) this only works based on the names of the objects. Images get a default name (numbering them). Shapes don’t. They aren’t selectable from the ‘Navigator’ therefore. However, shapes can get assigned a name explicitly and will then also be listed.

We might consider the lack of automatic naming a bug. On the other hand documents may contain dozens or even hundreds of shapes…

Surely a bug: A previously named shape that once was grouped and later made independent again does neither show its name nor show up in the ‘Navigator’. The original name is locked as used though not displayed. The shape doesn’t get accessible again if a new name is assigned.
Groups cannot be named at all.

Thanks @Lupp, sure a bug, images are showed even with with a default name, but now draw objects. There is a reported about Draw

Seems that objects are created without default name, so not showed on the navigator.

Since Draw shapes in a document tend to get very numerous, the exclusion of unnamed shapes from the navigator may be a blessing,in disguise. Definitely bad is the behaviour mentioned under “Surely a bug” and concerning former members of resolved groups. I did not yet serch for a related bug report.
BTW (just tested): in Calc shapes don’t get automatic names. In Writer they get. In Draw they even haven’t (shown?) such a property. Not a mess?