I've been sent a cycling top for an upcoming charity ride. It's a bit snug though - I can wear it, it's just a bit tight.
Any recommendations on how to stretch it?

I've heard wash it in cold water with fabric softener then stretch it - but I'm not sure if this will work with cycling tops.

Just start wearing it.

Don't use fabric softener on cycling "stuff". They won't last long if you do.

Get skinnier

It starts on Saturday - it's 160 mile ride and a 7 mile walk up a mountain in 24 hours - probably not the time to be cutting carbs

Sounds like fun!
I'm sure your top will get looser pretty quickly.

Thanks - saved me from making a mistake.

I bought some jerseys one size down as incentive to lose weight. I'm still working on it

Me? I'm with the Fool... Lol! Seriously, though, just stretch it a bit at the arms and belly. But if it restricts your breathing, you may want to reconsider using it--unless you absolutely have to.

Best way to wash cycling jerseys/bibs/gloves/headbands/socks/etc. is in cold water on gentle cycle with liquid soap, then hang up immediately to drip dry. Well...the socks I don't normally hang up. I just drape them over a folded up towel in the laundry room. The rest I hang in the doorways overnight. They are pretty dry by the morning.

Again, cold water, gentle cycle, liquid soap, hang immediately to drip dry. No softener, no regular cycle, no dryer, no "dry" soap. YMMV, works for me.

I quit doing that, my weight hasn't changed by 5 lbs in at least 10 years

Best plan is to exchange for a larger jersey, wearing a different jersey for the ride itself if need be.

Possibly a tailor could modify it if you have another jersey or wicking T to sacrifice with similar color and fabric. Basically combine two jerseys into one preserving the design elements from the event jersey and getting donor fabric from your other jersey.
It would be expensive, time-consuming, and results wouldn't be as good as having a jersey designed for your size.

ok so at least it wasn't just me, everyone found the shirts too small. So we had a bit of a switch around, with the "small" guys wearing "large", the "medium" guys wearing "XL" and the XL guys wearing cotton T-shirts just for the photos, and then their own tops for the ride itself.

The manufacturer are going to remake them anyway because they didn't come with the pockets they should have had.

Thanks for all the advice.