If you want to get a new band for your Apple Watch, the first step is to measure your wrist. You can do this by using a piece of string or a cloth tape measure. Once you have your measurement, you can insert it into the sizing tool on Apple's website to determine the correct size.
Generally, bands designed for smaller Apple Watches will fit a 40mm Series 6 watch. Likewise, bands designed for larger Apple Watches will fit a 38mm Series 6 watch. However, be sure to carefully remove the band from your watch when you are removing it. You don't want to have it slide back and forth while you're wearing it.
While all previous Apple Watch bands are compatible with the new series, be sure to order the correct size band. A 38mm link bracelet won't fit a 40mm Series 6 watch. Additionally, the finish on your new strap might not match the finish on your watch. If that happens, you can try using a paper tool to secure the strap. The final step is to check for uncomfortableness and make sure that your new band will fit properly.
Older Series 3 bands are compatible with the Series 6 and Series 7. However, they don't come in every size. The same goes for Series 6 and SE bands. The 38mm band could be too small for the 42mm version of the new Apple Watch. A 44mm band might be too wide, and vice versa.