Take it back to the dealer, they to disassemble the blade brake/clutch to replace the belt.

They just replaced the transmission and the belt

They could not replace the belt without removing the bbc. Obviously, it was not properly reassembled.

How do you replace the belt? Can you release the spring tension somehow?

Go back to the place that fixed it, jump up and down, and complain. If no response, wait for other customers to come in, then make a real fuss, and maybe they will fix it correctly!
They have evidently done something wrong. Maybe they gave a favorite nephew a Christmas time job, and his brain is messed up by too much mary j.
Do not attempt to fix it yourself! they can and will accuse you of doing something wrong to make them look bad, or some other excuse.
Most good dealers and repair shops do stand by their work, and will readily fix a goof-up, if it is their fault!
But some places like to sell lots of parts, and do so with no qualms, even if the part wasn't bad. There should be an "idler pulley" somewhere in the blade belt workings, that can be pushed back against the belt tension, to remove the blade belt. If you aren't mechanically inclined, best take it back to the guy who fixed it the first time.

Sounds like the Blade Brake Clutch is worn out doesnt it guys?

No- It sounds like the repair place screwed up!

Shops make mistakes, but the BBC unit does not have to be removed to change the belt.Just the plate on the side and the blade.
If it worked before without any problem then the cables got out of adjustment. This could happen if the handle was folded and somehow tangled when unfolded.
I really thing the cable is set wrong and that is all.
Like putting your foot on the clutch half way.
The sad thing is a lot of bashing Honda lawn equipment goes on this site.
I am 70 years young and have been working on lawn mowers since I was 12 years old. I like the Honda engines.
I remember when the Japanese motorcycles came to the USA and they were made fun of.
Well, they make a great product.
And yes, harder to work on, but Flat Head engines are antiques. What a waste a flat head engine.
Would you buy a new car with a flat head engine?