Changing the watchband of a cartier watch is simple and requires only one tool. A Cartier tank watch is rectangle-shaped; it was introduced in 1910 as a men’s watch. Through the 1930s, Cartier tanks were generally men’s watches, although smaller women's versions began to appear. A 12mm leather band is thin by contemporary watch fashion standards and is typically used with women’s timepieces. The leather band is held by two spring-loaded or screw-capped bars that are attached to the lugs. The lugs are the arms at the top and bottom of the watchcase that hold the spring bars in place.

Place the Cartier watch face down on a piece of cloth on a flat table. Insert a thin, flat blade, or a spring bar tool, between the edge of the leather band and the case lug. Push the blade against the spring bar shoulder, which is a tiny ledge near the end of the bar that allows you to push the spring bar down with the blade. Depress the spring by moving the blade slightly away from the lug. You will feel a tension spring. (Some models may have the shoulder in the center of the bar. The band will have an opening for access.)

Free the bar from the lug pinhole while keeping the spring bar depressed. You will see the tip of the spring bar come out of the hole. Push the bar away from the case lug pinhole with a finger. Use your forefinger and thumb to grasp the end of the spring bar and remove it from the end of the band. Repeat the procedure on the opposite side of the watch.

Use a cleaning cloth to wipe away any gunk, dirt or grit from the spring bar. Replace the spring bars if they are bent or if the spring is loose. A replacement bar costs a couple of dollars.

Insert the spring bars into the ends of the new leather band.The longer strap is attached to the 6 o'clock lugs and the shorter, buckled strap is mounted on the 12 o'clock lug. Insert one end of one spring bar into the case lug pinhole. Use the blade to depress the shoulder at the opposite end of the spring bar. Guide the spring bar, while still depressing the spring bar shoulder, into the second pinhole. Repeat the procedure on the opposite side of the watch.

Use a screwdriver to remove the screw head from the outside area of the lug near the tip. You'll see the screw head on the side of one lug only.

Loosen the head counterclockwise with the screwdriver from the case lug and let it fall onto the table. Remove the bar through the opposite lug. At the same time, the bar will come loose from the band.

Place the short portion of the new band between the 12 o'clock lugs and the long end between the 6 o'clock lugs. Align the bands with the lug pinholes. Thread the bars through one end of the lug pinhole and through the band ends. The other end of the bar will fit snugly into the opposite end pinhole.

Use a screwdriver to fasten the screw head into the outside of the lug pinhole until it tightens on the bar. Repeat procedure for the opposite side of the watch.