If your iPad is “stuck” on iOS 9.3.5, it is possible you have an older iPad 2, 3 or 1st gen iPad Mini that cannot be upgraded beyond iOS 9.3.5.
These older iDevices are actually 6 and 7 year old iDevices.
The internal hardware in these older iPads is too old and underpowered to sufficiently run newer iOS versions.
Also, if your iPad is slow,
Tap on the link below to my iDevice/iOS performance/optimisation user tips link.
Slow iPad on iOS 9, 10 or 11! - Apple Community
Try all of these tips and leave each tip active together. Do not simply try every tip individually, expecting just one tip to be a performance “cure all”.
Many of these tips are designed to work together to help improve overall iOS/iDevice performance.
Good Luck!