You can't "warm up" your knees, but you can keep them warm.

With clothing. My knees are best if I keep them near body temperature; ie well in to the 90s. I had made a pair of knee warmers that look more like the basketball knee pads of years ago. 2 layers of 300 eight wind-block fleece with two light elastic straps (light and not very tight to not constrain those hamstrings) and a "garter strap" of velcro up to "D" rings I sew into all my shorts and briefs. These get worn anytime it is mid-eighties or lower. Below high 70s I wear standard knee warmers over. Any tights or legwarmers go over all of this.

Those goofy knee warmers mean I don't have to overdress my legs to keep my knees warm, something I did for 20 years.

Ben