Kaleido. How do I reinstall back?

This will depend on the back of the watch. Totally smooth watch back is usually a push fit. Using a small precision screwdriver you Gently lever around the back plate until it 'Pops' off. the battery should hook out by CAREFULLY using the screwdriver you used to remove the back plate. *Note* check which way up the battery is fitted BEFORE you remove it. The back plate should just push on and should quietly click when seated correctly.
Some back plates are 'screw on'. You can usually spot these by the dimples or notches on the back plate where the removal tool fits.

Most watches have one of three types of rear cover. Push On, Screw On and Screw Down. The easiest way to tell is to look closely at the actual back cover. The batteries in some watches are held down with a spring loaded cover you unclip to access the
battery.
A totally smooth back cover suggests a Push On type. On the edge there should be a groove or a notch. Use a thin edged tool to lever off the cover with a firm but gently action. Once you have replaced the battery the cover should push on and click once in place.
If it has regular notches or dimples around the edge of the cover the whole cover will need to be unscrewed, the 'Screw On' type. The whole cover needs to turn like a bolt head. This often needs a specialist tool to do it.
The last type is a rear plate held down with four tiny screws. This is normally most common on 'Sport' watches.

For a quality watch like that I would recommend using a specialist watch repairer as they can reseal the watch to be water tight after the battery is replaced.
Could you possibly comment below with a photo of the back of the watch?
There are three main types of watch back plate. Push fit, screw down and screw on.
Push fit is usually a smooth back cover with No notches or grooves. These can be gently levered off to replace the battery then pushed down to 'click' back into place. Some have a slight notch on the body or back plate to make removal easier.
Screw down back plates have dimples or notches at regular places around the edge. This is to allow grip to unscrew the whole back plate. Some versions need a specialist tool to unscrew them.
Screw on plates use four tiny screws to hold the back plate down. These are more commonly used on 'Sports' or digital watches.

Some of these watches have a notch where the stem goes. If you don't have the back in place it will not snap back into place. Also some are a slight screw back and will snap off, but when putting them back you must kind of screw it back in place. Good luck.

man i have the same problem with the same watch did you ever find out what it was???

Try this method:-

place the watch on a flat surface (putting a cloth under it to avoidd scratching the face), place both thumbs on the back of the watch opposite the the notch.

Lean on your thumbs and work them around the edge until you reach the notch point and then increase the force until the back clicks into place.

Try this two or three times and if you still cannot get the back to click into place, then I suggest that you take it to a Boluva dealer (which I'm sure will cost you!) but it could be that a special press is needed to put it back on.

Good luck!

Depending on the age of the watch it could have a ECR series battery over the top of the 377 powering the movement or it could have 2 377's in the circuit board over 1 377 in the movement.

I always recommend taking Fossil watches to a certified repair shop that can re-seal the watch so you dont void your 12 year warranty and the watch remains water resistant and resistant to small particles.

Tools are available from many watch suppliers, one is Jules Borel, jborel.com, best to get a new gasket and mabey a spare if you wear this in the water then a new crown or crown seal also.

Your batteries are dying my friend, take it to pretty much anywhere that sells watches to have the battery replaced. there is a main battery for the watch itself and 2 smaller batteries that control the screen or face changing. I think even your local walmart can change the batteries for you.

Good luck!

Stay classy

B.

Pulsar battery replacement can be found at amazon, here: Rayovac Batteries Lasts as Long Costs Less. Best of luck