Resizing grandmothers ring

Alright y’all so I have the opportunity to wear my grandmothers anniversary band with my engagement ring and band that I already have. My grandmothers ring seems to be a tad big
but definitely not noticeable at all. But sometimes the diamonds will slide to the side and I’d just like them to stay upwards more without soldering the rings together. What places do y’all recommend for resizing and how long should it take!? Also, the ring is real gold.