My juniper started to look sick and is turning yellow and another one is turning brown.
I have replanted both bonsais in spring and used new bonsai soil I bought online from a bonsai specialized store, a premixed one.
We had pretty hot summer and I feel I have been underwatering the bonsai, especially, because the soil doesn't seem to hold water at all - the water drains through immediately, so I started to wonder if I have a good soil mix at all. It really looks like rocks, but the store seemed to be a real bonsai specialized store.

Any suggestions on how I can save my bonsai? Should I replant it to new soil mix? Should i cut out the branch?
The leaves are turning yellow only on one branch on one side, and I noticed it a couple of weeks ago.

Thank you

old needles die off, that is normal. Are the tips normal growing and healthy?

I haven't seen a lot of growth on this bonsai this year.
Most of the tree looks healthy (attaching the picture of a healthy looking branch), apart from one branch that looks sad (in the original post picture).

Do I need to remove the dead needles?

I wouldnt try to repot the tree especially if its not health. Furthermore the current soil looks fine and you want that type of substrate rather than soil. I would make sure to be feeding it regularly since there isnt anything for it to feed on in the substrate.

If you are keeping it where you took the picture then it will not do well, it needs to be outside. Junipers like sun.