how to adjust the mass of any object in solidworks?

Are you looking to retroactively assign a mass to a part? You can fudge it in the BOM for quick and dirty. I noticed you haven't assigned a material to the part you have attached. In that regard, you could assign any number of materials of varying densities to your part. Aluminium will have a total mass less than that of stainless which is less than that of uranium, etc.Are you shooting for something more exotic? You can add custom materials to the SW database if they aren't in there. Perhaps someone has a more extensive database?

You can also go to document properties and set the density in Material Properties (but that is old school, setting material is the way to go).

if you dont want to change the dimensions you can use it

if you have to change the dimensions, you have to make a Sensor and Parametric design,

See this Part please

It is an interesting question. assume you had buy an object like a DC motor and you only modeled it without details but you have data sheet and net weigh of that motor. so without choosing a specific material you want to add the mass of that object to your design. what is the solution?

Change your material, thats it to go