I am new to bonsai.
I just recieved a few bonsai from an online nursery. The trees seems to not be planted in proper bonsai soil. It looks like a combination of bonsai soil and regular potting soil.

Should I repot the bonsai with proper soil now (Mid summer) or wait until next spring?

I would wait till next Spring.
Could you please upload some pictures of them?
It helps.

Here are photos of 2 of my bonsai. Chinese elm and Golden Gate Ficus

Just posted photos. Thanks for your help

Let them get used to their new environment and repot only in Spring.
Take good care of the watering regimen.
Good luck!